| /* |
| * Copyright (c) 1998, 2015, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| * |
| */ |
| |
| #include "precompiled.hpp" |
| #include "classfile/vmSymbols.hpp" |
| #include "memory/resourceArea.hpp" |
| #include "oops/markOop.hpp" |
| #include "oops/oop.inline.hpp" |
| #include "runtime/handles.inline.hpp" |
| #include "runtime/interfaceSupport.hpp" |
| #include "runtime/mutexLocker.hpp" |
| #include "runtime/objectMonitor.hpp" |
| #include "runtime/objectMonitor.inline.hpp" |
| #include "runtime/orderAccess.inline.hpp" |
| #include "runtime/osThread.hpp" |
| #include "runtime/stubRoutines.hpp" |
| #include "runtime/thread.inline.hpp" |
| #include "services/threadService.hpp" |
| #include "trace/tracing.hpp" |
| #include "trace/traceMacros.hpp" |
| #include "utilities/dtrace.hpp" |
| #include "utilities/macros.hpp" |
| #include "utilities/preserveException.hpp" |
| #ifdef TARGET_OS_FAMILY_linux |
| # include "os_linux.inline.hpp" |
| #endif |
| #ifdef TARGET_OS_FAMILY_solaris |
| # include "os_solaris.inline.hpp" |
| #endif |
| #ifdef TARGET_OS_FAMILY_windows |
| # include "os_windows.inline.hpp" |
| #endif |
| #ifdef TARGET_OS_FAMILY_bsd |
| # include "os_bsd.inline.hpp" |
| #endif |
| |
| #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64) |
| // Need to inhibit inlining for older versions of GCC to avoid build-time failures |
| #define ATTR __attribute__((noinline)) |
| #else |
| #define ATTR |
| #endif |
| |
| |
| #ifdef DTRACE_ENABLED |
| |
| // Only bother with this argument setup if dtrace is available |
| // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. |
| |
| |
| #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ |
| char* bytes = NULL; \ |
| int len = 0; \ |
| jlong jtid = SharedRuntime::get_java_tid(thread); \ |
| Symbol* klassname = ((oop)obj)->klass()->name(); \ |
| if (klassname != NULL) { \ |
| bytes = (char*)klassname->bytes(); \ |
| len = klassname->utf8_length(); \ |
| } |
| |
| #ifndef USDT2 |
| |
| HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify, |
| jlong, uintptr_t, char*, int); |
| HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll, |
| jlong, uintptr_t, char*, int); |
| HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter, |
| jlong, uintptr_t, char*, int); |
| HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered, |
| jlong, uintptr_t, char*, int); |
| HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit, |
| jlong, uintptr_t, char*, int); |
| |
| #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ |
| { \ |
| if (DTraceMonitorProbes) { \ |
| DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \ |
| (monitor), bytes, len, (millis)); \ |
| } \ |
| } |
| |
| #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ |
| { \ |
| if (DTraceMonitorProbes) { \ |
| DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \ |
| (uintptr_t)(monitor), bytes, len); \ |
| } \ |
| } |
| |
| #else /* USDT2 */ |
| |
| #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ |
| { \ |
| if (DTraceMonitorProbes) { \ |
| DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| HOTSPOT_MONITOR_WAIT(jtid, \ |
| (monitor), bytes, len, (millis)); \ |
| } \ |
| } |
| |
| #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER |
| #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED |
| #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT |
| #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY |
| #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL |
| |
| #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ |
| { \ |
| if (DTraceMonitorProbes) { \ |
| DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ |
| HOTSPOT_MONITOR_##probe(jtid, \ |
| (uintptr_t)(monitor), bytes, len); \ |
| } \ |
| } |
| |
| #endif /* USDT2 */ |
| #else // ndef DTRACE_ENABLED |
| |
| #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} |
| #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} |
| |
| #endif // ndef DTRACE_ENABLED |
| |
| // Tunables ... |
| // The knob* variables are effectively final. Once set they should |
| // never be modified hence. Consider using __read_mostly with GCC. |
| |
| int ObjectMonitor::Knob_Verbose = 0 ; |
| int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool - |
| static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins |
| static int Knob_HandOff = 0 ; |
| static int Knob_ReportSettings = 0 ; |
| |
| static int Knob_SpinBase = 0 ; // Floor AKA SpinMin |
| static int Knob_SpinBackOff = 0 ; // spin-loop backoff |
| static int Knob_CASPenalty = -1 ; // Penalty for failed CAS |
| static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change |
| static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field |
| static int Knob_SpinEarly = 1 ; |
| static int Knob_SuccEnabled = 1 ; // futile wake throttling |
| static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one |
| static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs |
| static int Knob_Bonus = 100 ; // spin success bonus |
| static int Knob_BonusB = 100 ; // spin success bonus |
| static int Knob_Penalty = 200 ; // spin failure penalty |
| static int Knob_Poverty = 1000 ; |
| static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park() |
| static int Knob_FixedSpin = 0 ; |
| static int Knob_OState = 3 ; // Spinner checks thread state of _owner |
| static int Knob_UsePause = 1 ; |
| static int Knob_ExitPolicy = 0 ; |
| static int Knob_PreSpin = 10 ; // 20-100 likely better |
| static int Knob_ResetEvent = 0 ; |
| static int BackOffMask = 0 ; |
| |
| static int Knob_FastHSSEC = 0 ; |
| static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee |
| static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline |
| static volatile int InitDone = 0 ; |
| |
| #define TrySpin TrySpin_VaryDuration |
| |
| // ----------------------------------------------------------------------------- |
| // Theory of operations -- Monitors lists, thread residency, etc: |
| // |
| // * A thread acquires ownership of a monitor by successfully |
| // CAS()ing the _owner field from null to non-null. |
| // |
| // * Invariant: A thread appears on at most one monitor list -- |
| // cxq, EntryList or WaitSet -- at any one time. |
| // |
| // * Contending threads "push" themselves onto the cxq with CAS |
| // and then spin/park. |
| // |
| // * After a contending thread eventually acquires the lock it must |
| // dequeue itself from either the EntryList or the cxq. |
| // |
| // * The exiting thread identifies and unparks an "heir presumptive" |
| // tentative successor thread on the EntryList. Critically, the |
| // exiting thread doesn't unlink the successor thread from the EntryList. |
| // After having been unparked, the wakee will recontend for ownership of |
| // the monitor. The successor (wakee) will either acquire the lock or |
| // re-park itself. |
| // |
| // Succession is provided for by a policy of competitive handoff. |
| // The exiting thread does _not_ grant or pass ownership to the |
| // successor thread. (This is also referred to as "handoff" succession"). |
| // Instead the exiting thread releases ownership and possibly wakes |
| // a successor, so the successor can (re)compete for ownership of the lock. |
| // If the EntryList is empty but the cxq is populated the exiting |
| // thread will drain the cxq into the EntryList. It does so by |
| // by detaching the cxq (installing null with CAS) and folding |
| // the threads from the cxq into the EntryList. The EntryList is |
| // doubly linked, while the cxq is singly linked because of the |
| // CAS-based "push" used to enqueue recently arrived threads (RATs). |
| // |
| // * Concurrency invariants: |
| // |
| // -- only the monitor owner may access or mutate the EntryList. |
| // The mutex property of the monitor itself protects the EntryList |
| // from concurrent interference. |
| // -- Only the monitor owner may detach the cxq. |
| // |
| // * The monitor entry list operations avoid locks, but strictly speaking |
| // they're not lock-free. Enter is lock-free, exit is not. |
| // For a description of 'Methods and apparatus providing non-blocking access |
| // to a resource,' see U.S. Pat. No. 7844973. |
| // |
| // * The cxq can have multiple concurrent "pushers" but only one concurrent |
| // detaching thread. This mechanism is immune from the ABA corruption. |
| // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. |
| // |
| // * Taken together, the cxq and the EntryList constitute or form a |
| // single logical queue of threads stalled trying to acquire the lock. |
| // We use two distinct lists to improve the odds of a constant-time |
| // dequeue operation after acquisition (in the ::enter() epilog) and |
| // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). |
| // A key desideratum is to minimize queue & monitor metadata manipulation |
| // that occurs while holding the monitor lock -- that is, we want to |
| // minimize monitor lock holds times. Note that even a small amount of |
| // fixed spinning will greatly reduce the # of enqueue-dequeue operations |
| // on EntryList|cxq. That is, spinning relieves contention on the "inner" |
| // locks and monitor metadata. |
| // |
| // Cxq points to the the set of Recently Arrived Threads attempting entry. |
| // Because we push threads onto _cxq with CAS, the RATs must take the form of |
| // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when |
| // the unlocking thread notices that EntryList is null but _cxq is != null. |
| // |
| // The EntryList is ordered by the prevailing queue discipline and |
| // can be organized in any convenient fashion, such as a doubly-linked list or |
| // a circular doubly-linked list. Critically, we want insert and delete operations |
| // to operate in constant-time. If we need a priority queue then something akin |
| // to Solaris' sleepq would work nicely. Viz., |
| // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. |
| // Queue discipline is enforced at ::exit() time, when the unlocking thread |
| // drains the cxq into the EntryList, and orders or reorders the threads on the |
| // EntryList accordingly. |
| // |
| // Barring "lock barging", this mechanism provides fair cyclic ordering, |
| // somewhat similar to an elevator-scan. |
| // |
| // * The monitor synchronization subsystem avoids the use of native |
| // synchronization primitives except for the narrow platform-specific |
| // park-unpark abstraction. See the comments in os_solaris.cpp regarding |
| // the semantics of park-unpark. Put another way, this monitor implementation |
| // depends only on atomic operations and park-unpark. The monitor subsystem |
| // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the |
| // underlying OS manages the READY<->RUN transitions. |
| // |
| // * Waiting threads reside on the WaitSet list -- wait() puts |
| // the caller onto the WaitSet. |
| // |
| // * notify() or notifyAll() simply transfers threads from the WaitSet to |
| // either the EntryList or cxq. Subsequent exit() operations will |
| // unpark the notifyee. Unparking a notifee in notify() is inefficient - |
| // it's likely the notifyee would simply impale itself on the lock held |
| // by the notifier. |
| // |
| // * An interesting alternative is to encode cxq as (List,LockByte) where |
| // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary |
| // variable, like _recursions, in the scheme. The threads or Events that form |
| // the list would have to be aligned in 256-byte addresses. A thread would |
| // try to acquire the lock or enqueue itself with CAS, but exiting threads |
| // could use a 1-0 protocol and simply STB to set the LockByte to 0. |
| // Note that is is *not* word-tearing, but it does presume that full-word |
| // CAS operations are coherent with intermix with STB operations. That's true |
| // on most common processors. |
| // |
| // * See also http://blogs.sun.com/dave |
| |
| |
| // ----------------------------------------------------------------------------- |
| // Enter support |
| |
| bool ObjectMonitor::try_enter(Thread* THREAD) { |
| if (THREAD != _owner) { |
| if (THREAD->is_lock_owned ((address)_owner)) { |
| assert(_recursions == 0, "internal state error"); |
| _owner = THREAD ; |
| _recursions = 1 ; |
| OwnerIsThread = 1 ; |
| return true; |
| } |
| if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { |
| return false; |
| } |
| return true; |
| } else { |
| _recursions++; |
| return true; |
| } |
| } |
| |
| void ATTR ObjectMonitor::enter(TRAPS) { |
| // The following code is ordered to check the most common cases first |
| // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. |
| Thread * const Self = THREAD ; |
| void * cur ; |
| |
| cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; |
| if (cur == NULL) { |
| // Either ASSERT _recursions == 0 or explicitly set _recursions = 0. |
| assert (_recursions == 0 , "invariant") ; |
| assert (_owner == Self, "invariant") ; |
| // CONSIDER: set or assert OwnerIsThread == 1 |
| return ; |
| } |
| |
| if (cur == Self) { |
| // TODO-FIXME: check for integer overflow! BUGID 6557169. |
| _recursions ++ ; |
| return ; |
| } |
| |
| if (Self->is_lock_owned ((address)cur)) { |
| assert (_recursions == 0, "internal state error"); |
| _recursions = 1 ; |
| // Commute owner from a thread-specific on-stack BasicLockObject address to |
| // a full-fledged "Thread *". |
| _owner = Self ; |
| OwnerIsThread = 1 ; |
| return ; |
| } |
| |
| // We've encountered genuine contention. |
| assert (Self->_Stalled == 0, "invariant") ; |
| Self->_Stalled = intptr_t(this) ; |
| |
| // Try one round of spinning *before* enqueueing Self |
| // and before going through the awkward and expensive state |
| // transitions. The following spin is strictly optional ... |
| // Note that if we acquire the monitor from an initial spin |
| // we forgo posting JVMTI events and firing DTRACE probes. |
| if (Knob_SpinEarly && TrySpin (Self) > 0) { |
| assert (_owner == Self , "invariant") ; |
| assert (_recursions == 0 , "invariant") ; |
| assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; |
| Self->_Stalled = 0 ; |
| return ; |
| } |
| |
| assert (_owner != Self , "invariant") ; |
| assert (_succ != Self , "invariant") ; |
| assert (Self->is_Java_thread() , "invariant") ; |
| JavaThread * jt = (JavaThread *) Self ; |
| assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ; |
| assert (jt->thread_state() != _thread_blocked , "invariant") ; |
| assert (this->object() != NULL , "invariant") ; |
| assert (_count >= 0, "invariant") ; |
| |
| // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy(). |
| // Ensure the object-monitor relationship remains stable while there's contention. |
| Atomic::inc_ptr(&_count); |
| |
| EventJavaMonitorEnter event; |
| |
| { // Change java thread status to indicate blocked on monitor enter. |
| JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); |
| |
| DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); |
| if (JvmtiExport::should_post_monitor_contended_enter()) { |
| JvmtiExport::post_monitor_contended_enter(jt, this); |
| |
| // The current thread does not yet own the monitor and does not |
| // yet appear on any queues that would get it made the successor. |
| // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event |
| // handler cannot accidentally consume an unpark() meant for the |
| // ParkEvent associated with this ObjectMonitor. |
| } |
| |
| OSThreadContendState osts(Self->osthread()); |
| ThreadBlockInVM tbivm(jt); |
| |
| Self->set_current_pending_monitor(this); |
| |
| // TODO-FIXME: change the following for(;;) loop to straight-line code. |
| for (;;) { |
| jt->set_suspend_equivalent(); |
| // cleared by handle_special_suspend_equivalent_condition() |
| // or java_suspend_self() |
| |
| EnterI (THREAD) ; |
| |
| if (!ExitSuspendEquivalent(jt)) break ; |
| |
| // |
| // We have acquired the contended monitor, but while we were |
| // waiting another thread suspended us. We don't want to enter |
| // the monitor while suspended because that would surprise the |
| // thread that suspended us. |
| // |
| _recursions = 0 ; |
| _succ = NULL ; |
| exit (false, Self) ; |
| |
| jt->java_suspend_self(); |
| } |
| Self->set_current_pending_monitor(NULL); |
| |
| // We cleared the pending monitor info since we've just gotten past |
| // the enter-check-for-suspend dance and we now own the monitor free |
| // and clear, i.e., it is no longer pending. The ThreadBlockInVM |
| // destructor can go to a safepoint at the end of this block. If we |
| // do a thread dump during that safepoint, then this thread will show |
| // as having "-locked" the monitor, but the OS and java.lang.Thread |
| // states will still report that the thread is blocked trying to |
| // acquire it. |
| } |
| |
| Atomic::dec_ptr(&_count); |
| assert (_count >= 0, "invariant") ; |
| Self->_Stalled = 0 ; |
| |
| // Must either set _recursions = 0 or ASSERT _recursions == 0. |
| assert (_recursions == 0 , "invariant") ; |
| assert (_owner == Self , "invariant") ; |
| assert (_succ != Self , "invariant") ; |
| assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; |
| |
| // The thread -- now the owner -- is back in vm mode. |
| // Report the glorious news via TI,DTrace and jvmstat. |
| // The probe effect is non-trivial. All the reportage occurs |
| // while we hold the monitor, increasing the length of the critical |
| // section. Amdahl's parallel speedup law comes vividly into play. |
| // |
| // Another option might be to aggregate the events (thread local or |
| // per-monitor aggregation) and defer reporting until a more opportune |
| // time -- such as next time some thread encounters contention but has |
| // yet to acquire the lock. While spinning that thread could |
| // spinning we could increment JVMStat counters, etc. |
| |
| DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); |
| if (JvmtiExport::should_post_monitor_contended_entered()) { |
| JvmtiExport::post_monitor_contended_entered(jt, this); |
| |
| // The current thread already owns the monitor and is not going to |
| // call park() for the remainder of the monitor enter protocol. So |
| // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED |
| // event handler consumed an unpark() issued by the thread that |
| // just exited the monitor. |
| } |
| |
| if (event.should_commit()) { |
| event.set_klass(((oop)this->object())->klass()); |
| event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid); |
| event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr())); |
| event.commit(); |
| } |
| |
| if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) { |
| ObjectMonitor::_sync_ContendedLockAttempts->inc() ; |
| } |
| } |
| |
| |
| // Caveat: TryLock() is not necessarily serializing if it returns failure. |
| // Callers must compensate as needed. |
| |
| int ObjectMonitor::TryLock (Thread * Self) { |
| for (;;) { |
| void * own = _owner ; |
| if (own != NULL) return 0 ; |
| if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) { |
| // Either guarantee _recursions == 0 or set _recursions = 0. |
| assert (_recursions == 0, "invariant") ; |
| assert (_owner == Self, "invariant") ; |
| // CONSIDER: set or assert that OwnerIsThread == 1 |
| return 1 ; |
| } |
| // The lock had been free momentarily, but we lost the race to the lock. |
| // Interference -- the CAS failed. |
| // We can either return -1 or retry. |
| // Retry doesn't make as much sense because the lock was just acquired. |
| if (true) return -1 ; |
| } |
| } |
| |
| void ATTR ObjectMonitor::EnterI (TRAPS) { |
| Thread * Self = THREAD ; |
| assert (Self->is_Java_thread(), "invariant") ; |
| assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ; |
| |
| // Try the lock - TATAS |
| if (TryLock (Self) > 0) { |
| assert (_succ != Self , "invariant") ; |
| assert (_owner == Self , "invariant") ; |
| assert (_Responsible != Self , "invariant") ; |
| return ; |
| } |
| |
| DeferredInitialize () ; |
| |
| // We try one round of spinning *before* enqueueing Self. |
| // |
| // If the _owner is ready but OFFPROC we could use a YieldTo() |
| // operation to donate the remainder of this thread's quantum |
| // to the owner. This has subtle but beneficial affinity |
| // effects. |
| |
| if (TrySpin (Self) > 0) { |
| assert (_owner == Self , "invariant") ; |
| assert (_succ != Self , "invariant") ; |
| assert (_Responsible != Self , "invariant") ; |
| return ; |
| } |
| |
| // The Spin failed -- Enqueue and park the thread ... |
| assert (_succ != Self , "invariant") ; |
| assert (_owner != Self , "invariant") ; |
| assert (_Responsible != Self , "invariant") ; |
| |
| // Enqueue "Self" on ObjectMonitor's _cxq. |
| // |
| // Node acts as a proxy for Self. |
| // As an aside, if were to ever rewrite the synchronization code mostly |
| // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class |
| // Java objects. This would avoid awkward lifecycle and liveness issues, |
| // as well as eliminate a subset of ABA issues. |
| // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. |
| // |
| |
| ObjectWaiter node(Self) ; |
| Self->_ParkEvent->reset() ; |
| node._prev = (ObjectWaiter *) 0xBAD ; |
| node.TState = ObjectWaiter::TS_CXQ ; |
| |
| // Push "Self" onto the front of the _cxq. |
| // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. |
| // Note that spinning tends to reduce the rate at which threads |
| // enqueue and dequeue on EntryList|cxq. |
| ObjectWaiter * nxt ; |
| for (;;) { |
| node._next = nxt = _cxq ; |
| if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ; |
| |
| // Interference - the CAS failed because _cxq changed. Just retry. |
| // As an optional optimization we retry the lock. |
| if (TryLock (Self) > 0) { |
| assert (_succ != Self , "invariant") ; |
| assert (_owner == Self , "invariant") ; |
| assert (_Responsible != Self , "invariant") ; |
| return ; |
| } |
| } |
| |
| // Check for cxq|EntryList edge transition to non-null. This indicates |
| // the onset of contention. While contention persists exiting threads |
| // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit |
| // operations revert to the faster 1-0 mode. This enter operation may interleave |
| // (race) a concurrent 1-0 exit operation, resulting in stranding, so we |
| // arrange for one of the contending thread to use a timed park() operations |
| // to detect and recover from the race. (Stranding is form of progress failure |
| // where the monitor is unlocked but all the contending threads remain parked). |
| // That is, at least one of the contended threads will periodically poll _owner. |
| // One of the contending threads will become the designated "Responsible" thread. |
| // The Responsible thread uses a timed park instead of a normal indefinite park |
| // operation -- it periodically wakes and checks for and recovers from potential |
| // strandings admitted by 1-0 exit operations. We need at most one Responsible |
| // thread per-monitor at any given moment. Only threads on cxq|EntryList may |
| // be responsible for a monitor. |
| // |
| // Currently, one of the contended threads takes on the added role of "Responsible". |
| // A viable alternative would be to use a dedicated "stranding checker" thread |
| // that periodically iterated over all the threads (or active monitors) and unparked |
| // successors where there was risk of stranding. This would help eliminate the |
| // timer scalability issues we see on some platforms as we'd only have one thread |
| // -- the checker -- parked on a timer. |
| |
| if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) { |
| // Try to assume the role of responsible thread for the monitor. |
| // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } |
| Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; |
| } |
| |
| // The lock have been released while this thread was occupied queueing |
| // itself onto _cxq. To close the race and avoid "stranding" and |
| // progress-liveness failure we must resample-retry _owner before parking. |
| // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. |
| // In this case the ST-MEMBAR is accomplished with CAS(). |
| // |
| // TODO: Defer all thread state transitions until park-time. |
| // Since state transitions are heavy and inefficient we'd like |
| // to defer the state transitions until absolutely necessary, |
| // and in doing so avoid some transitions ... |
| |
| TEVENT (Inflated enter - Contention) ; |
| int nWakeups = 0 ; |
| int RecheckInterval = 1 ; |
| |
| for (;;) { |
| |
| if (TryLock (Self) > 0) break ; |
| assert (_owner != Self, "invariant") ; |
| |
| if ((SyncFlags & 2) && _Responsible == NULL) { |
| Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; |
| } |
| |
| // park self |
| if (_Responsible == Self || (SyncFlags & 1)) { |
| TEVENT (Inflated enter - park TIMED) ; |
| Self->_ParkEvent->park ((jlong) RecheckInterval) ; |
| // Increase the RecheckInterval, but clamp the value. |
| RecheckInterval *= 8 ; |
| if (RecheckInterval > 1000) RecheckInterval = 1000 ; |
| } else { |
| TEVENT (Inflated enter - park UNTIMED) ; |
| Self->_ParkEvent->park() ; |
| } |
| |
| if (TryLock(Self) > 0) break ; |
| |
| // The lock is still contested. |
| // Keep a tally of the # of futile wakeups. |
| // Note that the counter is not protected by a lock or updated by atomics. |
| // That is by design - we trade "lossy" counters which are exposed to |
| // races during updates for a lower probe effect. |
| TEVENT (Inflated enter - Futile wakeup) ; |
| if (ObjectMonitor::_sync_FutileWakeups != NULL) { |
| ObjectMonitor::_sync_FutileWakeups->inc() ; |
| } |
| ++ nWakeups ; |
| |
| // Assuming this is not a spurious wakeup we'll normally find _succ == Self. |
| // We can defer clearing _succ until after the spin completes |
| // TrySpin() must tolerate being called with _succ == Self. |
| // Try yet another round of adaptive spinning. |
| if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ; |
| |
| // We can find that we were unpark()ed and redesignated _succ while |
| // we were spinning. That's harmless. If we iterate and call park(), |
| // park() will consume the event and return immediately and we'll |
| // just spin again. This pattern can repeat, leaving _succ to simply |
| // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks(). |
| // Alternately, we can sample fired() here, and if set, forgo spinning |
| // in the next iteration. |
| |
| if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) { |
| Self->_ParkEvent->reset() ; |
| OrderAccess::fence() ; |
| } |
| if (_succ == Self) _succ = NULL ; |
| |
| // Invariant: after clearing _succ a thread *must* retry _owner before parking. |
| OrderAccess::fence() ; |
| } |
| |
| // Egress : |
| // Self has acquired the lock -- Unlink Self from the cxq or EntryList. |
| // Normally we'll find Self on the EntryList . |
| // From the perspective of the lock owner (this thread), the |
| // EntryList is stable and cxq is prepend-only. |
| // The head of cxq is volatile but the interior is stable. |
| // In addition, Self.TState is stable. |
| |
| assert (_owner == Self , "invariant") ; |
| assert (object() != NULL , "invariant") ; |
| // I'd like to write: |
| // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; |
| // but as we're at a safepoint that's not safe. |
| |
| UnlinkAfterAcquire (Self, &node) ; |
| if (_succ == Self) _succ = NULL ; |
| |
| assert (_succ != Self, "invariant") ; |
| if (_Responsible == Self) { |
| _Responsible = NULL ; |
| OrderAccess::fence(); // Dekker pivot-point |
| |
| // We may leave threads on cxq|EntryList without a designated |
| // "Responsible" thread. This is benign. When this thread subsequently |
| // exits the monitor it can "see" such preexisting "old" threads -- |
| // threads that arrived on the cxq|EntryList before the fence, above -- |
| // by LDing cxq|EntryList. Newly arrived threads -- that is, threads |
| // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible |
| // non-null and elect a new "Responsible" timer thread. |
| // |
| // This thread executes: |
| // ST Responsible=null; MEMBAR (in enter epilog - here) |
| // LD cxq|EntryList (in subsequent exit) |
| // |
| // Entering threads in the slow/contended path execute: |
| // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) |
| // The (ST cxq; MEMBAR) is accomplished with CAS(). |
| // |
| // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent |
| // exit operation from floating above the ST Responsible=null. |
| } |
| |
| // We've acquired ownership with CAS(). |
| // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. |
| // But since the CAS() this thread may have also stored into _succ, |
| // EntryList, cxq or Responsible. These meta-data updates must be |
| // visible __before this thread subsequently drops the lock. |
| // Consider what could occur if we didn't enforce this constraint -- |
| // STs to monitor meta-data and user-data could reorder with (become |
| // visible after) the ST in exit that drops ownership of the lock. |
| // Some other thread could then acquire the lock, but observe inconsistent |
| // or old monitor meta-data and heap data. That violates the JMM. |
| // To that end, the 1-0 exit() operation must have at least STST|LDST |
| // "release" barrier semantics. Specifically, there must be at least a |
| // STST|LDST barrier in exit() before the ST of null into _owner that drops |
| // the lock. The barrier ensures that changes to monitor meta-data and data |
| // protected by the lock will be visible before we release the lock, and |
| // therefore before some other thread (CPU) has a chance to acquire the lock. |
| // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. |
| // |
| // Critically, any prior STs to _succ or EntryList must be visible before |
| // the ST of null into _owner in the *subsequent* (following) corresponding |
| // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily |
| // execute a serializing instruction. |
| |
| if (SyncFlags & 8) { |
| OrderAccess::fence() ; |
| } |
| return ; |
| } |
| |
| // ReenterI() is a specialized inline form of the latter half of the |
| // contended slow-path from EnterI(). We use ReenterI() only for |
| // monitor reentry in wait(). |
| // |
| // In the future we should reconcile EnterI() and ReenterI(), adding |
| // Knob_Reset and Knob_SpinAfterFutile support and restructuring the |
| // loop accordingly. |
| |
| void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) { |
| assert (Self != NULL , "invariant") ; |
| assert (SelfNode != NULL , "invariant") ; |
| assert (SelfNode->_thread == Self , "invariant") ; |
| assert (_waiters > 0 , "invariant") ; |
| assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ; |
| assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ; |
| JavaThread * jt = (JavaThread *) Self ; |
| |
| int nWakeups = 0 ; |
| for (;;) { |
| ObjectWaiter::TStates v = SelfNode->TState ; |
| guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; |
| assert (_owner != Self, "invariant") ; |
| |
| if (TryLock (Self) > 0) break ; |
| if (TrySpin (Self) > 0) break ; |
| |
| TEVENT (Wait Reentry - parking) ; |
| |
| // State transition wrappers around park() ... |
| // ReenterI() wisely defers state transitions until |
| // it's clear we must park the thread. |
| { |
| OSThreadContendState osts(Self->osthread()); |
| ThreadBlockInVM tbivm(jt); |
| |
| // cleared by handle_special_suspend_equivalent_condition() |
| // or java_suspend_self() |
| jt->set_suspend_equivalent(); |
| if (SyncFlags & 1) { |
| Self->_ParkEvent->park ((jlong)1000) ; |
| } else { |
| Self->_ParkEvent->park () ; |
| } |
| |
| // were we externally suspended while we were waiting? |
| for (;;) { |
| if (!ExitSuspendEquivalent (jt)) break ; |
| if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } |
| jt->java_suspend_self(); |
| jt->set_suspend_equivalent(); |
| } |
| } |
| |
| // Try again, but just so we distinguish between futile wakeups and |
| // successful wakeups. The following test isn't algorithmically |
| // necessary, but it helps us maintain sensible statistics. |
| if (TryLock(Self) > 0) break ; |
| |
| // The lock is still contested. |
| // Keep a tally of the # of futile wakeups. |
| // Note that the counter is not protected by a lock or updated by atomics. |
| // That is by design - we trade "lossy" counters which are exposed to |
| // races during updates for a lower probe effect. |
| TEVENT (Wait Reentry - futile wakeup) ; |
| ++ nWakeups ; |
| |
| // Assuming this is not a spurious wakeup we'll normally |
| // find that _succ == Self. |
| if (_succ == Self) _succ = NULL ; |
| |
| // Invariant: after clearing _succ a contending thread |
| // *must* retry _owner before parking. |
| OrderAccess::fence() ; |
| |
| if (ObjectMonitor::_sync_FutileWakeups != NULL) { |
| ObjectMonitor::_sync_FutileWakeups->inc() ; |
| } |
| } |
| |
| // Self has acquired the lock -- Unlink Self from the cxq or EntryList . |
| // Normally we'll find Self on the EntryList. |
| // Unlinking from the EntryList is constant-time and atomic-free. |
| // From the perspective of the lock owner (this thread), the |
| // EntryList is stable and cxq is prepend-only. |
| // The head of cxq is volatile but the interior is stable. |
| // In addition, Self.TState is stable. |
| |
| assert (_owner == Self, "invariant") ; |
| assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; |
| UnlinkAfterAcquire (Self, SelfNode) ; |
| if (_succ == Self) _succ = NULL ; |
| assert (_succ != Self, "invariant") ; |
| SelfNode->TState = ObjectWaiter::TS_RUN ; |
| OrderAccess::fence() ; // see comments at the end of EnterI() |
| } |
| |
| // after the thread acquires the lock in ::enter(). Equally, we could defer |
| // unlinking the thread until ::exit()-time. |
| |
| void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode) |
| { |
| assert (_owner == Self, "invariant") ; |
| assert (SelfNode->_thread == Self, "invariant") ; |
| |
| if (SelfNode->TState == ObjectWaiter::TS_ENTER) { |
| // Normal case: remove Self from the DLL EntryList . |
| // This is a constant-time operation. |
| ObjectWaiter * nxt = SelfNode->_next ; |
| ObjectWaiter * prv = SelfNode->_prev ; |
| if (nxt != NULL) nxt->_prev = prv ; |
| if (prv != NULL) prv->_next = nxt ; |
| if (SelfNode == _EntryList ) _EntryList = nxt ; |
| assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ; |
| assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ; |
| TEVENT (Unlink from EntryList) ; |
| } else { |
| guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ; |
| // Inopportune interleaving -- Self is still on the cxq. |
| // This usually means the enqueue of self raced an exiting thread. |
| // Normally we'll find Self near the front of the cxq, so |
| // dequeueing is typically fast. If needbe we can accelerate |
| // this with some MCS/CHL-like bidirectional list hints and advisory |
| // back-links so dequeueing from the interior will normally operate |
| // in constant-time. |
| // Dequeue Self from either the head (with CAS) or from the interior |
| // with a linear-time scan and normal non-atomic memory operations. |
| // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList |
| // and then unlink Self from EntryList. We have to drain eventually, |
| // so it might as well be now. |
| |
| ObjectWaiter * v = _cxq ; |
| assert (v != NULL, "invariant") ; |
| if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) { |
| // The CAS above can fail from interference IFF a "RAT" arrived. |
| // In that case Self must be in the interior and can no longer be |
| // at the head of cxq. |
| if (v == SelfNode) { |
| assert (_cxq != v, "invariant") ; |
| v = _cxq ; // CAS above failed - start scan at head of list |
| } |
| ObjectWaiter * p ; |
| ObjectWaiter * q = NULL ; |
| for (p = v ; p != NULL && p != SelfNode; p = p->_next) { |
| q = p ; |
| assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ; |
| } |
| assert (v != SelfNode, "invariant") ; |
| assert (p == SelfNode, "Node not found on cxq") ; |
| assert (p != _cxq, "invariant") ; |
| assert (q != NULL, "invariant") ; |
| assert (q->_next == p, "invariant") ; |
| q->_next = p->_next ; |
| } |
| TEVENT (Unlink from cxq) ; |
| } |
| |
| // Diagnostic hygiene ... |
| SelfNode->_prev = (ObjectWaiter *) 0xBAD ; |
| SelfNode->_next = (ObjectWaiter *) 0xBAD ; |
| SelfNode->TState = ObjectWaiter::TS_RUN ; |
| } |
| |
| // ----------------------------------------------------------------------------- |
| // Exit support |
| // |
| // exit() |
| // ~~~~~~ |
| // Note that the collector can't reclaim the objectMonitor or deflate |
| // the object out from underneath the thread calling ::exit() as the |
| // thread calling ::exit() never transitions to a stable state. |
| // This inhibits GC, which in turn inhibits asynchronous (and |
| // inopportune) reclamation of "this". |
| // |
| // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; |
| // There's one exception to the claim above, however. EnterI() can call |
| // exit() to drop a lock if the acquirer has been externally suspended. |
| // In that case exit() is called with _thread_state as _thread_blocked, |
| // but the monitor's _count field is > 0, which inhibits reclamation. |
| // |
| // 1-0 exit |
| // ~~~~~~~~ |
| // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of |
| // the fast-path operators have been optimized so the common ::exit() |
| // operation is 1-0. See i486.ad fast_unlock(), for instance. |
| // The code emitted by fast_unlock() elides the usual MEMBAR. This |
| // greatly improves latency -- MEMBAR and CAS having considerable local |
| // latency on modern processors -- but at the cost of "stranding". Absent the |
| // MEMBAR, a thread in fast_unlock() can race a thread in the slow |
| // ::enter() path, resulting in the entering thread being stranding |
| // and a progress-liveness failure. Stranding is extremely rare. |
| // We use timers (timed park operations) & periodic polling to detect |
| // and recover from stranding. Potentially stranded threads periodically |
| // wake up and poll the lock. See the usage of the _Responsible variable. |
| // |
| // The CAS() in enter provides for safety and exclusion, while the CAS or |
| // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking |
| // eliminates the CAS/MEMBAR from the exist path, but it admits stranding. |
| // We detect and recover from stranding with timers. |
| // |
| // If a thread transiently strands it'll park until (a) another |
| // thread acquires the lock and then drops the lock, at which time the |
| // exiting thread will notice and unpark the stranded thread, or, (b) |
| // the timer expires. If the lock is high traffic then the stranding latency |
| // will be low due to (a). If the lock is low traffic then the odds of |
| // stranding are lower, although the worst-case stranding latency |
| // is longer. Critically, we don't want to put excessive load in the |
| // platform's timer subsystem. We want to minimize both the timer injection |
| // rate (timers created/sec) as well as the number of timers active at |
| // any one time. (more precisely, we want to minimize timer-seconds, which is |
| // the integral of the # of active timers at any instant over time). |
| // Both impinge on OS scalability. Given that, at most one thread parked on |
| // a monitor will use a timer. |
| |
| void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) { |
| Thread * Self = THREAD ; |
| if (THREAD != _owner) { |
| if (THREAD->is_lock_owned((address) _owner)) { |
| // Transmute _owner from a BasicLock pointer to a Thread address. |
| // We don't need to hold _mutex for this transition. |
| // Non-null to Non-null is safe as long as all readers can |
| // tolerate either flavor. |
| assert (_recursions == 0, "invariant") ; |
| _owner = THREAD ; |
| _recursions = 0 ; |
| OwnerIsThread = 1 ; |
| } else { |
| // NOTE: we need to handle unbalanced monitor enter/exit |
| // in native code by throwing an exception. |
| // TODO: Throw an IllegalMonitorStateException ? |
| TEVENT (Exit - Throw IMSX) ; |
| assert(false, "Non-balanced monitor enter/exit!"); |
| if (false) { |
| THROW(vmSymbols::java_lang_IllegalMonitorStateException()); |
| } |
| return; |
| } |
| } |
| |
| if (_recursions != 0) { |
| _recursions--; // this is simple recursive enter |
| TEVENT (Inflated exit - recursive) ; |
| return ; |
| } |
| |
| // Invariant: after setting Responsible=null an thread must execute |
| // a MEMBAR or other serializing instruction before fetching EntryList|cxq. |
| if ((SyncFlags & 4) == 0) { |
| _Responsible = NULL ; |
| } |
| |
| #if INCLUDE_TRACE |
| // get the owner's thread id for the MonitorEnter event |
| // if it is enabled and the thread isn't suspended |
| if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) { |
| _previous_owner_tid = SharedRuntime::get_java_tid(Self); |
| } |
| #endif |
| |
| for (;;) { |
| assert (THREAD == _owner, "invariant") ; |
| |
| |
| if (Knob_ExitPolicy == 0) { |
| // release semantics: prior loads and stores from within the critical section |
| // must not float (reorder) past the following store that drops the lock. |
| // On SPARC that requires MEMBAR #loadstore|#storestore. |
| // But of course in TSO #loadstore|#storestore is not required. |
| // I'd like to write one of the following: |
| // A. OrderAccess::release() ; _owner = NULL |
| // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL; |
| // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both |
| // store into a _dummy variable. That store is not needed, but can result |
| // in massive wasteful coherency traffic on classic SMP systems. |
| // Instead, I use release_store(), which is implemented as just a simple |
| // ST on x64, x86 and SPARC. |
| OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock |
| OrderAccess::storeload() ; // See if we need to wake a successor |
| if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { |
| TEVENT (Inflated exit - simple egress) ; |
| return ; |
| } |
| TEVENT (Inflated exit - complex egress) ; |
| |
| // Normally the exiting thread is responsible for ensuring succession, |
| // but if other successors are ready or other entering threads are spinning |
| // then this thread can simply store NULL into _owner and exit without |
| // waking a successor. The existence of spinners or ready successors |
| // guarantees proper succession (liveness). Responsibility passes to the |
| // ready or running successors. The exiting thread delegates the duty. |
| // More precisely, if a successor already exists this thread is absolved |
| // of the responsibility of waking (unparking) one. |
| // |
| // The _succ variable is critical to reducing futile wakeup frequency. |
| // _succ identifies the "heir presumptive" thread that has been made |
| // ready (unparked) but that has not yet run. We need only one such |
| // successor thread to guarantee progress. |
| // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf |
| // section 3.3 "Futile Wakeup Throttling" for details. |
| // |
| // Note that spinners in Enter() also set _succ non-null. |
| // In the current implementation spinners opportunistically set |
| // _succ so that exiting threads might avoid waking a successor. |
| // Another less appealing alternative would be for the exiting thread |
| // to drop the lock and then spin briefly to see if a spinner managed |
| // to acquire the lock. If so, the exiting thread could exit |
| // immediately without waking a successor, otherwise the exiting |
| // thread would need to dequeue and wake a successor. |
| // (Note that we'd need to make the post-drop spin short, but no |
| // shorter than the worst-case round-trip cache-line migration time. |
| // The dropped lock needs to become visible to the spinner, and then |
| // the acquisition of the lock by the spinner must become visible to |
| // the exiting thread). |
| // |
| |
| // It appears that an heir-presumptive (successor) must be made ready. |
| // Only the current lock owner can manipulate the EntryList or |
| // drain _cxq, so we need to reacquire the lock. If we fail |
| // to reacquire the lock the responsibility for ensuring succession |
| // falls to the new owner. |
| // |
| if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { |
| return ; |
| } |
| TEVENT (Exit - Reacquired) ; |
| } else { |
| if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { |
| OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock |
| OrderAccess::storeload() ; |
| // Ratify the previously observed values. |
| if (_cxq == NULL || _succ != NULL) { |
| TEVENT (Inflated exit - simple egress) ; |
| return ; |
| } |
| |
| // inopportune interleaving -- the exiting thread (this thread) |
| // in the fast-exit path raced an entering thread in the slow-enter |
| // path. |
| // We have two choices: |
| // A. Try to reacquire the lock. |
| // If the CAS() fails return immediately, otherwise |
| // we either restart/rerun the exit operation, or simply |
| // fall-through into the code below which wakes a successor. |
| // B. If the elements forming the EntryList|cxq are TSM |
| // we could simply unpark() the lead thread and return |
| // without having set _succ. |
| if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { |
| TEVENT (Inflated exit - reacquired succeeded) ; |
| return ; |
| } |
| TEVENT (Inflated exit - reacquired failed) ; |
| } else { |
| TEVENT (Inflated exit - complex egress) ; |
| } |
| } |
| |
| guarantee (_owner == THREAD, "invariant") ; |
| |
| ObjectWaiter * w = NULL ; |
| int QMode = Knob_QMode ; |
| |
| if (QMode == 2 && _cxq != NULL) { |
| // QMode == 2 : cxq has precedence over EntryList. |
| // Try to directly wake a successor from the cxq. |
| // If successful, the successor will need to unlink itself from cxq. |
| w = _cxq ; |
| assert (w != NULL, "invariant") ; |
| assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ; |
| ExitEpilog (Self, w) ; |
| return ; |
| } |
| |
| if (QMode == 3 && _cxq != NULL) { |
| // Aggressively drain cxq into EntryList at the first opportunity. |
| // This policy ensure that recently-run threads live at the head of EntryList. |
| // Drain _cxq into EntryList - bulk transfer. |
| // First, detach _cxq. |
| // The following loop is tantamount to: w = swap (&cxq, NULL) |
| w = _cxq ; |
| for (;;) { |
| assert (w != NULL, "Invariant") ; |
| ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; |
| if (u == w) break ; |
| w = u ; |
| } |
| assert (w != NULL , "invariant") ; |
| |
| ObjectWaiter * q = NULL ; |
| ObjectWaiter * p ; |
| for (p = w ; p != NULL ; p = p->_next) { |
| guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; |
| p->TState = ObjectWaiter::TS_ENTER ; |
| p->_prev = q ; |
| q = p ; |
| } |
| |
| // Append the RATs to the EntryList |
| // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time. |
| ObjectWaiter * Tail ; |
| for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ; |
| if (Tail == NULL) { |
| _EntryList = w ; |
| } else { |
| Tail->_next = w ; |
| w->_prev = Tail ; |
| } |
| |
| // Fall thru into code that tries to wake a successor from EntryList |
| } |
| |
| if (QMode == 4 && _cxq != NULL) { |
| // Aggressively drain cxq into EntryList at the first opportunity. |
| // This policy ensure that recently-run threads live at the head of EntryList. |
| |
| // Drain _cxq into EntryList - bulk transfer. |
| // First, detach _cxq. |
| // The following loop is tantamount to: w = swap (&cxq, NULL) |
| w = _cxq ; |
| for (;;) { |
| assert (w != NULL, "Invariant") ; |
| ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; |
| if (u == w) break ; |
| w = u ; |
| } |
| assert (w != NULL , "invariant") ; |
| |
| ObjectWaiter * q = NULL ; |
| ObjectWaiter * p ; |
| for (p = w ; p != NULL ; p = p->_next) { |
| guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; |
| p->TState = ObjectWaiter::TS_ENTER ; |
| p->_prev = q ; |
| q = p ; |
| } |
| |
| // Prepend the RATs to the EntryList |
| if (_EntryList != NULL) { |
| q->_next = _EntryList ; |
| _EntryList->_prev = q ; |
| } |
| _EntryList = w ; |
| |
| // Fall thru into code that tries to wake a successor from EntryList |
| } |
| |
| w = _EntryList ; |
| if (w != NULL) { |
| // I'd like to write: guarantee (w->_thread != Self). |
| // But in practice an exiting thread may find itself on the EntryList. |
| // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and |
| // then calls exit(). Exit release the lock by setting O._owner to NULL. |
| // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The |
| // notify() operation moves T1 from O's waitset to O's EntryList. T2 then |
| // release the lock "O". T2 resumes immediately after the ST of null into |
| // _owner, above. T2 notices that the EntryList is populated, so it |
| // reacquires the lock and then finds itself on the EntryList. |
| // Given all that, we have to tolerate the circumstance where "w" is |
| // associated with Self. |
| assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; |
| ExitEpilog (Self, w) ; |
| return ; |
| } |
| |
| // If we find that both _cxq and EntryList are null then just |
| // re-run the exit protocol from the top. |
| w = _cxq ; |
| if (w == NULL) continue ; |
| |
| // Drain _cxq into EntryList - bulk transfer. |
| // First, detach _cxq. |
| // The following loop is tantamount to: w = swap (&cxq, NULL) |
| for (;;) { |
| assert (w != NULL, "Invariant") ; |
| ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; |
| if (u == w) break ; |
| w = u ; |
| } |
| TEVENT (Inflated exit - drain cxq into EntryList) ; |
| |
| assert (w != NULL , "invariant") ; |
| assert (_EntryList == NULL , "invariant") ; |
| |
| // Convert the LIFO SLL anchored by _cxq into a DLL. |
| // The list reorganization step operates in O(LENGTH(w)) time. |
| // It's critical that this step operate quickly as |
| // "Self" still holds the outer-lock, restricting parallelism |
| // and effectively lengthening the critical section. |
| // Invariant: s chases t chases u. |
| // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so |
| // we have faster access to the tail. |
| |
| if (QMode == 1) { |
| // QMode == 1 : drain cxq to EntryList, reversing order |
| // We also reverse the order of the list. |
| ObjectWaiter * s = NULL ; |
| ObjectWaiter * t = w ; |
| ObjectWaiter * u = NULL ; |
| while (t != NULL) { |
| guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ; |
| t->TState = ObjectWaiter::TS_ENTER ; |
| u = t->_next ; |
| t->_prev = u ; |
| t->_next = s ; |
| s = t; |
| t = u ; |
| } |
| _EntryList = s ; |
| assert (s != NULL, "invariant") ; |
| } else { |
| // QMode == 0 or QMode == 2 |
| _EntryList = w ; |
| ObjectWaiter * q = NULL ; |
| ObjectWaiter * p ; |
| for (p = w ; p != NULL ; p = p->_next) { |
| guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; |
| p->TState = ObjectWaiter::TS_ENTER ; |
| p->_prev = q ; |
| q = p ; |
| } |
| } |
| |
| // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL |
| // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). |
| |
| // See if we can abdicate to a spinner instead of waking a thread. |
| // A primary goal of the implementation is to reduce the |
| // context-switch rate. |
| if (_succ != NULL) continue; |
| |
| w = _EntryList ; |
| if (w != NULL) { |
| guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; |
| ExitEpilog (Self, w) ; |
| return ; |
| } |
| } |
| } |
| |
| // ExitSuspendEquivalent: |
| // A faster alternate to handle_special_suspend_equivalent_condition() |
| // |
| // handle_special_suspend_equivalent_condition() unconditionally |
| // acquires the SR_lock. On some platforms uncontended MutexLocker() |
| // operations have high latency. Note that in ::enter() we call HSSEC |
| // while holding the monitor, so we effectively lengthen the critical sections. |
| // |
| // There are a number of possible solutions: |
| // |
| // A. To ameliorate the problem we might also defer state transitions |
| // to as late as possible -- just prior to parking. |
| // Given that, we'd call HSSEC after having returned from park(), |
| // but before attempting to acquire the monitor. This is only a |
| // partial solution. It avoids calling HSSEC while holding the |
| // monitor (good), but it still increases successor reacquisition latency -- |
| // the interval between unparking a successor and the time the successor |
| // resumes and retries the lock. See ReenterI(), which defers state transitions. |
| // If we use this technique we can also avoid EnterI()-exit() loop |
| // in ::enter() where we iteratively drop the lock and then attempt |
| // to reacquire it after suspending. |
| // |
| // B. In the future we might fold all the suspend bits into a |
| // composite per-thread suspend flag and then update it with CAS(). |
| // Alternately, a Dekker-like mechanism with multiple variables |
| // would suffice: |
| // ST Self->_suspend_equivalent = false |
| // MEMBAR |
| // LD Self_>_suspend_flags |
| // |
| |
| |
| bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) { |
| int Mode = Knob_FastHSSEC ; |
| if (Mode && !jSelf->is_external_suspend()) { |
| assert (jSelf->is_suspend_equivalent(), "invariant") ; |
| jSelf->clear_suspend_equivalent() ; |
| if (2 == Mode) OrderAccess::storeload() ; |
| if (!jSelf->is_external_suspend()) return false ; |
| // We raced a suspension -- fall thru into the slow path |
| TEVENT (ExitSuspendEquivalent - raced) ; |
| jSelf->set_suspend_equivalent() ; |
| } |
| return jSelf->handle_special_suspend_equivalent_condition() ; |
| } |
| |
| |
| void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) { |
| assert (_owner == Self, "invariant") ; |
| |
| // Exit protocol: |
| // 1. ST _succ = wakee |
| // 2. membar #loadstore|#storestore; |
| // 2. ST _owner = NULL |
| // 3. unpark(wakee) |
| |
| _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ; |
| ParkEvent * Trigger = Wakee->_event ; |
| |
| // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. |
| // The thread associated with Wakee may have grabbed the lock and "Wakee" may be |
| // out-of-scope (non-extant). |
| Wakee = NULL ; |
| |
| // Drop the lock |
| OrderAccess::release_store_ptr (&_owner, NULL) ; |
| OrderAccess::fence() ; // ST _owner vs LD in unpark() |
| |
| if (SafepointSynchronize::do_call_back()) { |
| TEVENT (unpark before SAFEPOINT) ; |
| } |
| |
| DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); |
| Trigger->unpark() ; |
| |
| // Maintain stats and report events to JVMTI |
| if (ObjectMonitor::_sync_Parks != NULL) { |
| ObjectMonitor::_sync_Parks->inc() ; |
| } |
| } |
| |
| |
| // ----------------------------------------------------------------------------- |
| // Class Loader deadlock handling. |
| // |
| // complete_exit exits a lock returning recursion count |
| // complete_exit/reenter operate as a wait without waiting |
| // complete_exit requires an inflated monitor |
| // The _owner field is not always the Thread addr even with an |
| // inflated monitor, e.g. the monitor can be inflated by a non-owning |
| // thread due to contention. |
| intptr_t ObjectMonitor::complete_exit(TRAPS) { |
| Thread * const Self = THREAD; |
| assert(Self->is_Java_thread(), "Must be Java thread!"); |
| JavaThread *jt = (JavaThread *)THREAD; |
| |
| DeferredInitialize(); |
| |
| if (THREAD != _owner) { |
| if (THREAD->is_lock_owned ((address)_owner)) { |
| assert(_recursions == 0, "internal state error"); |
| _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ |
| _recursions = 0 ; |
| OwnerIsThread = 1 ; |
| } |
| } |
| |
| guarantee(Self == _owner, "complete_exit not owner"); |
| intptr_t save = _recursions; // record the old recursion count |
| _recursions = 0; // set the recursion level to be 0 |
| exit (true, Self) ; // exit the monitor |
| guarantee (_owner != Self, "invariant"); |
| return save; |
| } |
| |
| // reenter() enters a lock and sets recursion count |
| // complete_exit/reenter operate as a wait without waiting |
| void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { |
| Thread * const Self = THREAD; |
| assert(Self->is_Java_thread(), "Must be Java thread!"); |
| JavaThread *jt = (JavaThread *)THREAD; |
| |
| guarantee(_owner != Self, "reenter already owner"); |
| enter (THREAD); // enter the monitor |
| guarantee (_recursions == 0, "reenter recursion"); |
| _recursions = recursions; |
| return; |
| } |
| |
| |
| // ----------------------------------------------------------------------------- |
| // A macro is used below because there may already be a pending |
| // exception which should not abort the execution of the routines |
| // which use this (which is why we don't put this into check_slow and |
| // call it with a CHECK argument). |
| |
| #define CHECK_OWNER() \ |
| do { \ |
| if (THREAD != _owner) { \ |
| if (THREAD->is_lock_owned((address) _owner)) { \ |
| _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \ |
| _recursions = 0; \ |
| OwnerIsThread = 1 ; \ |
| } else { \ |
| TEVENT (Throw IMSX) ; \ |
| THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \ |
| } \ |
| } \ |
| } while (false) |
| |
| // check_slow() is a misnomer. It's called to simply to throw an IMSX exception. |
| // TODO-FIXME: remove check_slow() -- it's likely dead. |
| |
| void ObjectMonitor::check_slow(TRAPS) { |
| TEVENT (check_slow - throw IMSX) ; |
| assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner"); |
| THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner"); |
| } |
| |
| static int Adjust (volatile int * adr, int dx) { |
| int v ; |
| for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ; |
| return v ; |
| } |
| |
| // helper method for posting a monitor wait event |
| void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event, |
| jlong notifier_tid, |
| jlong timeout, |
| bool timedout) { |
| event->set_klass(((oop)this->object())->klass()); |
| event->set_timeout((TYPE_ULONG)timeout); |
| event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr())); |
| event->set_notifier((TYPE_OSTHREAD)notifier_tid); |
| event->set_timedOut((TYPE_BOOLEAN)timedout); |
| event->commit(); |
| } |
| |
| // ----------------------------------------------------------------------------- |
| // Wait/Notify/NotifyAll |
| // |
| // Note: a subset of changes to ObjectMonitor::wait() |
| // will need to be replicated in complete_exit above |
| void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { |
| Thread * const Self = THREAD ; |
| assert(Self->is_Java_thread(), "Must be Java thread!"); |
| JavaThread *jt = (JavaThread *)THREAD; |
| |
| DeferredInitialize () ; |
| |
| // Throw IMSX or IEX. |
| CHECK_OWNER(); |
| |
| EventJavaMonitorWait event; |
| |
| // check for a pending interrupt |
| if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { |
| // post monitor waited event. Note that this is past-tense, we are done waiting. |
| if (JvmtiExport::should_post_monitor_waited()) { |
| // Note: 'false' parameter is passed here because the |
| // wait was not timed out due to thread interrupt. |
| JvmtiExport::post_monitor_waited(jt, this, false); |
| |
| // In this short circuit of the monitor wait protocol, the |
| // current thread never drops ownership of the monitor and |
| // never gets added to the wait queue so the current thread |
| // cannot be made the successor. This means that the |
| // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally |
| // consume an unpark() meant for the ParkEvent associated with |
| // this ObjectMonitor. |
| } |
| if (event.should_commit()) { |
| post_monitor_wait_event(&event, 0, millis, false); |
| } |
| TEVENT (Wait - Throw IEX) ; |
| THROW(vmSymbols::java_lang_InterruptedException()); |
| return ; |
| } |
| |
| TEVENT (Wait) ; |
| |
| assert (Self->_Stalled == 0, "invariant") ; |
| Self->_Stalled = intptr_t(this) ; |
| jt->set_current_waiting_monitor(this); |
| |
| // create a node to be put into the queue |
| // Critically, after we reset() the event but prior to park(), we must check |
| // for a pending interrupt. |
| ObjectWaiter node(Self); |
| node.TState = ObjectWaiter::TS_WAIT ; |
| Self->_ParkEvent->reset() ; |
| OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag |
| |
| // Enter the waiting queue, which is a circular doubly linked list in this case |
| // but it could be a priority queue or any data structure. |
| // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only |
| // by the the owner of the monitor *except* in the case where park() |
| // returns because of a timeout of interrupt. Contention is exceptionally rare |
| // so we use a simple spin-lock instead of a heavier-weight blocking lock. |
| |
| Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ; |
| AddWaiter (&node) ; |
| Thread::SpinRelease (&_WaitSetLock) ; |
| |
| if ((SyncFlags & 4) == 0) { |
| _Responsible = NULL ; |
| } |
| intptr_t save = _recursions; // record the old recursion count |
| _waiters++; // increment the number of waiters |
| _recursions = 0; // set the recursion level to be 1 |
| exit (true, Self) ; // exit the monitor |
| guarantee (_owner != Self, "invariant") ; |
| |
| // The thread is on the WaitSet list - now park() it. |
| // On MP systems it's conceivable that a brief spin before we park |
| // could be profitable. |
| // |
| // TODO-FIXME: change the following logic to a loop of the form |
| // while (!timeout && !interrupted && _notified == 0) park() |
| |
| int ret = OS_OK ; |
| int WasNotified = 0 ; |
| { // State transition wrappers |
| OSThread* osthread = Self->osthread(); |
| OSThreadWaitState osts(osthread, true); |
| { |
| ThreadBlockInVM tbivm(jt); |
| // Thread is in thread_blocked state and oop access is unsafe. |
| jt->set_suspend_equivalent(); |
| |
| if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) { |
| // Intentionally empty |
| } else |
| if (node._notified == 0) { |
| if (millis <= 0) { |
| Self->_ParkEvent->park () ; |
| } else { |
| ret = Self->_ParkEvent->park (millis) ; |
| } |
| } |
| |
| // were we externally suspended while we were waiting? |
| if (ExitSuspendEquivalent (jt)) { |
| // TODO-FIXME: add -- if succ == Self then succ = null. |
| jt->java_suspend_self(); |
| } |
| |
| } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm |
| |
| |
| // Node may be on the WaitSet, the EntryList (or cxq), or in transition |
| // from the WaitSet to the EntryList. |
| // See if we need to remove Node from the WaitSet. |
| // We use double-checked locking to avoid grabbing _WaitSetLock |
| // if the thread is not on the wait queue. |
| // |
| // Note that we don't need a fence before the fetch of TState. |
| // In the worst case we'll fetch a old-stale value of TS_WAIT previously |
| // written by the is thread. (perhaps the fetch might even be satisfied |
| // by a look-aside into the processor's own store buffer, although given |
| // the length of the code path between the prior ST and this load that's |
| // highly unlikely). If the following LD fetches a stale TS_WAIT value |
| // then we'll acquire the lock and then re-fetch a fresh TState value. |
| // That is, we fail toward safety. |
| |
| if (node.TState == ObjectWaiter::TS_WAIT) { |
| Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ; |
| if (node.TState == ObjectWaiter::TS_WAIT) { |
| DequeueSpecificWaiter (&node) ; // unlink from WaitSet |
| assert(node._notified == 0, "invariant"); |
| node.TState = ObjectWaiter::TS_RUN ; |
| } |
| Thread::SpinRelease (&_WaitSetLock) ; |
| } |
| |
| // The thread is now either on off-list (TS_RUN), |
| // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). |
| // The Node's TState variable is stable from the perspective of this thread. |
| // No other threads will asynchronously modify TState. |
| guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ; |
| OrderAccess::loadload() ; |
| if (_succ == Self) _succ = NULL ; |
| WasNotified = node._notified ; |
| |
| // Reentry phase -- reacquire the monitor. |
| // re-enter contended monitor after object.wait(). |
| // retain OBJECT_WAIT state until re-enter successfully completes |
| // Thread state is thread_in_vm and oop access is again safe, |
| // although the raw address of the object may have changed. |
| // (Don't cache naked oops over safepoints, of course). |
| |
| // post monitor waited event. Note that this is past-tense, we are done waiting. |
| if (JvmtiExport::should_post_monitor_waited()) { |
| JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); |
| |
| if (node._notified != 0 && _succ == Self) { |
| // In this part of the monitor wait-notify-reenter protocol it |
| // is possible (and normal) for another thread to do a fastpath |
| // monitor enter-exit while this thread is still trying to get |
| // to the reenter portion of the protocol. |
| // |
| // The ObjectMonitor was notified and the current thread is |
| // the successor which also means that an unpark() has already |
| // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can |
| // consume the unpark() that was done when the successor was |
| // set because the same ParkEvent is shared between Java |
| // monitors and JVM/TI RawMonitors (for now). |
| // |
| // We redo the unpark() to ensure forward progress, i.e., we |
| // don't want all pending threads hanging (parked) with none |
| // entering the unlocked monitor. |
| node._event->unpark(); |
| } |
| } |
| |
| if (event.should_commit()) { |
| post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT); |
| } |
| |
| OrderAccess::fence() ; |
| |
| assert (Self->_Stalled != 0, "invariant") ; |
| Self->_Stalled = 0 ; |
| |
| assert (_owner != Self, "invariant") ; |
| ObjectWaiter::TStates v = node.TState ; |
| if (v == ObjectWaiter::TS_RUN) { |
| enter (Self) ; |
| } else { |
| guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; |
| ReenterI (Self, &node) ; |
| node.wait_reenter_end(this); |
| } |
| |
| // Self has reacquired the lock. |
| // Lifecycle - the node representing Self must not appear on any queues. |
| // Node is about to go out-of-scope, but even if it were immortal we wouldn't |
| // want residual elements associated with this thread left on any lists. |
| guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ; |
| assert (_owner == Self, "invariant") ; |
| assert (_succ != Self , "invariant") ; |
| } // OSThreadWaitState() |
| |
| jt->set_current_waiting_monitor(NULL); |
| |
| guarantee (_recursions == 0, "invariant") ; |
| _recursions = save; // restore the old recursion count |
| _waiters--; // decrement the number of waiters |
| |
| // Verify a few postconditions |
| assert (_owner == Self , "invariant") ; |
| assert (_succ != Self , "invariant") ; |
| assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; |
| |
| if (SyncFlags & 32) { |
| OrderAccess::fence() ; |
| } |
| |
| // check if the notification happened |
| if (!WasNotified) { |
| // no, it could be timeout or Thread.interrupt() or both |
| // check for interrupt event, otherwise it is timeout |
| if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { |
| TEVENT (Wait - throw IEX from epilog) ; |
| THROW(vmSymbols::java_lang_InterruptedException()); |
| } |
| } |
| |
| // NOTE: Spurious wake up will be consider as timeout. |
| // Monitor notify has precedence over thread interrupt. |
| } |
| |
| |
| // Consider: |
| // If the lock is cool (cxq == null && succ == null) and we're on an MP system |
| // then instead of transferring a thread from the WaitSet to the EntryList |
| // we might just dequeue a thread from the WaitSet and directly unpark() it. |
| |
| void ObjectMonitor::notify(TRAPS) { |
| CHECK_OWNER(); |
| if (_WaitSet == NULL) { |
| TEVENT (Empty-Notify) ; |
| return ; |
| } |
| DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); |
| |
| int Policy = Knob_MoveNotifyee ; |
| |
| Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ; |
| ObjectWaiter * iterator = DequeueWaiter() ; |
| if (iterator != NULL) { |
| TEVENT (Notify1 - Transfer) ; |
| guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; |
| guarantee (iterator->_notified == 0, "invariant") ; |
| if (Policy != 4) { |
| iterator->TState = ObjectWaiter::TS_ENTER ; |
| } |
| iterator->_notified = 1 ; |
| Thread * Self = THREAD; |
| iterator->_notifier_tid = Self->osthread()->thread_id(); |
| |
| ObjectWaiter * List = _EntryList ; |
| if (List != NULL) { |
| assert (List->_prev == NULL, "invariant") ; |
| assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; |
| assert (List != iterator, "invariant") ; |
| } |
| |
| if (Policy == 0) { // prepend to EntryList |
| if (List == NULL) { |
| iterator->_next = iterator->_prev = NULL ; |
| _EntryList = iterator ; |
| } else { |
| List->_prev = iterator ; |
| iterator->_next = List ; |
| iterator->_prev = NULL ; |
| _EntryList = iterator ; |
| } |
| } else |
| if (Policy == 1) { // append to EntryList |
| if (List == NULL) { |
| iterator->_next = iterator->_prev = NULL ; |
| _EntryList = iterator ; |
| } else { |
| // CONSIDER: finding the tail currently requires a linear-time walk of |
| // the EntryList. We can make tail access constant-time by converting to |
| // a CDLL instead of using our current DLL. |
| ObjectWaiter * Tail ; |
| for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; |
| assert (Tail != NULL && Tail->_next == NULL, "invariant") ; |
| Tail->_next = iterator ; |
| iterator->_prev = Tail ; |
| iterator->_next = NULL ; |
| } |
| } else |
| if (Policy == 2) { // prepend to cxq |
| // prepend to cxq |
| if (List == NULL) { |
| iterator->_next = iterator->_prev = NULL ; |
| _EntryList = iterator ; |
| } else { |
| iterator->TState = ObjectWaiter::TS_CXQ ; |
| for (;;) { |
| ObjectWaiter * Front = _cxq ; |
| iterator->_next = Front ; |
| if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { |
| break ; |
| } |
| } |
| } |
| } else |
| if (Policy == 3) { // append to cxq |
| iterator->TState = ObjectWaiter::TS_CXQ ; |
| for (;;) { |
| ObjectWaiter * Tail ; |
| Tail = _cxq ; |
| if (Tail == NULL) { |
| iterator->_next = NULL ; |
| if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { |
| break ; |
| } |
| } else { |
| while (Tail->_next != NULL) Tail = Tail->_next ; |
| Tail->_next = iterator ; |
| iterator->_prev = Tail ; |
| iterator->_next = NULL ; |
| break ; |
| } |
| } |
| } else { |
| ParkEvent * ev = iterator->_event ; |
| iterator->TState = ObjectWaiter::TS_RUN ; |
| OrderAccess::fence() ; |
| ev->unpark() ; |
| } |
| |
| if (Policy < 4) { |
| iterator->wait_reenter_begin(this); |
| } |
| |
| // _WaitSetLock protects the wait queue, not the EntryList. We could |
| // move the add-to-EntryList operation, above, outside the critical section |
| // protected by _WaitSetLock. In practice that's not useful. With the |
| // exception of wait() timeouts and interrupts the monitor owner |
| // is the only thread that grabs _WaitSetLock. There's almost no contention |
| // on _WaitSetLock so it's not profitable to reduce the length of the |
| // critical section. |
| } |
| |
| Thread::SpinRelease (&_WaitSetLock) ; |
| |
| if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) { |
| ObjectMonitor::_sync_Notifications->inc() ; |
| } |
| } |
| |
| |
| void ObjectMonitor::notifyAll(TRAPS) { |
| CHECK_OWNER(); |
| ObjectWaiter* iterator; |
| if (_WaitSet == NULL) { |
| TEVENT (Empty-NotifyAll) ; |
| return ; |
| } |
| DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); |
| |
| int Policy = Knob_MoveNotifyee ; |
| int Tally = 0 ; |
| Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ; |
| |
| for (;;) { |
| iterator = DequeueWaiter () ; |
| if (iterator == NULL) break ; |
| TEVENT (NotifyAll - Transfer1) ; |
| ++Tally ; |
| |
| // Disposition - what might we do with iterator ? |
| // a. add it directly to the EntryList - either tail or head. |
| // b. push it onto the front of the _cxq. |
| // For now we use (a). |
| |
| guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; |
| guarantee (iterator->_notified == 0, "invariant") ; |
| iterator->_notified = 1 ; |
| Thread * Self = THREAD; |
| iterator->_notifier_tid = Self->osthread()->thread_id(); |
| if (Policy != 4) { |
| iterator->TState = ObjectWaiter::TS_ENTER ; |
| } |
| |
| ObjectWaiter * List = _EntryList ; |
| if (List != NULL) { |
| assert (List->_prev == NULL, "invariant") ; |
| assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; |
| assert (List != iterator, "invariant") ; |
| } |
| |
| if (Policy == 0) { // prepend to EntryList |
| if (List == NULL) { |
| iterator->_next = iterator->_prev = NULL ; |
| _EntryList = iterator ; |
| } else { |
| List->_prev = iterator ; |
| iterator->_next = List ; |
| iterator->_prev = NULL ; |
| _EntryList = iterator ; |
| } |
| } else |
| if (Policy == 1) { // append to EntryList |
| if (List == NULL) { |
| iterator->_next = iterator->_prev = NULL ; |
| _EntryList = iterator ; |
| } else { |
| // CONSIDER: finding the tail currently requires a linear-time walk of |
| // the EntryList. We can make tail access constant-time by converting to |
| // a CDLL instead of using our current DLL. |
| ObjectWaiter * Tail ; |
| for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; |
| assert (Tail != NULL && Tail->_next == NULL, "invariant") ; |
| Tail->_next = iterator ; |
| iterator->_prev = Tail ; |
| iterator->_next = NULL ; |
| } |
| } else |
| if (Policy == 2) { // prepend to cxq |
| // prepend to cxq |
| iterator->TState = ObjectWaiter::TS_CXQ ; |
| for (;;) { |
| ObjectWaiter * Front = _cxq ; |
| iterator->_next = Front ; |
| if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { |
| break ; |
| } |
| } |
| } else |
| if (Policy == 3) { // append to cxq |
| iterator->TState = ObjectWaiter::TS_CXQ ; |
| for (;;) { |
| ObjectWaiter * Tail ; |
| Tail = _cxq ; |
| if (Tail == NULL) { |
| iterator->_next = NULL ; |
| if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { |
| break ; |
| } |
| } else { |
| while (Tail->_next != NULL) Tail = Tail->_next ; |
| Tail->_next = iterator ; |
| iterator->_prev = Tail ; |
| iterator->_next = NULL ; |
| break ; |
| } |
| } |
| } else { |
| ParkEvent * ev = iterator->_event ; |
| iterator->TState = ObjectWaiter::TS_RUN ; |
| OrderAccess::fence() ; |
| ev->unpark() ; |
| } |
| |
| if (Policy < 4) { |
| iterator->wait_reenter_begin(this); |
| } |
| |
| // _WaitSetLock protects the wait queue, not the EntryList. We could |
| // move the add-to-EntryList operation, above, outside the critical section |
| // protected by _WaitSetLock. In practice that's not useful. With the |
| // exception of wait() timeouts and interrupts the monitor owner |
| // is the only thread that grabs _WaitSetLock. There's almost no contention |
| // on _WaitSetLock so it's not profitable to reduce the length of the |
| // critical section. |
| } |
| |
| Thread::SpinRelease (&_WaitSetLock) ; |
| |
| if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) { |
| ObjectMonitor::_sync_Notifications->inc(Tally) ; |
| } |
| } |
| |
| // ----------------------------------------------------------------------------- |
| // Adaptive Spinning Support |
| // |
| // Adaptive spin-then-block - rational spinning |
| // |
| // Note that we spin "globally" on _owner with a classic SMP-polite TATAS |
| // algorithm. On high order SMP systems it would be better to start with |
| // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, |
| // a contending thread could enqueue itself on the cxq and then spin locally |
| // on a thread-specific variable such as its ParkEvent._Event flag. |
| // That's left as an exercise for the reader. Note that global spinning is |
| // not problematic on Niagara, as the L2$ serves the interconnect and has both |
| // low latency and massive bandwidth. |
| // |
| // Broadly, we can fix the spin frequency -- that is, the % of contended lock |
| // acquisition attempts where we opt to spin -- at 100% and vary the spin count |
| // (duration) or we can fix the count at approximately the duration of |
| // a context switch and vary the frequency. Of course we could also |
| // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. |
| // For a description of 'Adaptive spin-then-block mutual exclusion in |
| // multi-threaded processing,' see U.S. Pat. No. 8046758. |
| // |
| // This implementation varies the duration "D", where D varies with |
| // the success rate of recent spin attempts. (D is capped at approximately |
| // length of a round-trip context switch). The success rate for recent |
| // spin attempts is a good predictor of the success rate of future spin |
| // attempts. The mechanism adapts automatically to varying critical |
| // section length (lock modality), system load and degree of parallelism. |
| // D is maintained per-monitor in _SpinDuration and is initialized |
| // optimistically. Spin frequency is fixed at 100%. |
| // |
| // Note that _SpinDuration is volatile, but we update it without locks |
| // or atomics. The code is designed so that _SpinDuration stays within |
| // a reasonable range even in the presence of races. The arithmetic |
| // operations on _SpinDuration are closed over the domain of legal values, |
| // so at worst a race will install and older but still legal value. |
| // At the very worst this introduces some apparent non-determinism. |
| // We might spin when we shouldn't or vice-versa, but since the spin |
| // count are relatively short, even in the worst case, the effect is harmless. |
| // |
| // Care must be taken that a low "D" value does not become an |
| // an absorbing state. Transient spinning failures -- when spinning |
| // is overall profitable -- should not cause the system to converge |
| // on low "D" values. We want spinning to be stable and predictable |
| // and fairly responsive to change and at the same time we don't want |
| // it to oscillate, become metastable, be "too" non-deterministic, |
| // or converge on or enter undesirable stable absorbing states. |
| // |
| // We implement a feedback-based control system -- using past behavior |
| // to predict future behavior. We face two issues: (a) if the |
| // input signal is random then the spin predictor won't provide optimal |
| // results, and (b) if the signal frequency is too high then the control |
| // system, which has some natural response lag, will "chase" the signal. |
| // (b) can arise from multimodal lock hold times. Transient preemption |
| // can also result in apparent bimodal lock hold times. |
| // Although sub-optimal, neither condition is particularly harmful, as |
| // in the worst-case we'll spin when we shouldn't or vice-versa. |
| // The maximum spin duration is rather short so the failure modes aren't bad. |
| // To be conservative, I've tuned the gain in system to bias toward |
| // _not spinning. Relatedly, the system can sometimes enter a mode where it |
| // "rings" or oscillates between spinning and not spinning. This happens |
| // when spinning is just on the cusp of profitability, however, so the |
| // situation is not dire. The state is benign -- there's no need to add |
| // hysteresis control to damp the transition rate between spinning and |
| // not spinning. |
| // |
| |
| intptr_t ObjectMonitor::SpinCallbackArgument = 0 ; |
| int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ; |
| |
| // Spinning: Fixed frequency (100%), vary duration |
| |
| |
| int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) { |
| |
| // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. |
| int ctr = Knob_FixedSpin ; |
| if (ctr != 0) { |
| while (--ctr >= 0) { |
| if (TryLock (Self) > 0) return 1 ; |
| SpinPause () ; |
| } |
| return 0 ; |
| } |
| |
| for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) { |
| if (TryLock(Self) > 0) { |
| // Increase _SpinDuration ... |
| // Note that we don't clamp SpinDuration precisely at SpinLimit. |
| // Raising _SpurDuration to the poverty line is key. |
| int x = _SpinDuration ; |
| if (x < Knob_SpinLimit) { |
| if (x < Knob_Poverty) x = Knob_Poverty ; |
| _SpinDuration = x + Knob_BonusB ; |
| } |
| return 1 ; |
| } |
| SpinPause () ; |
| } |
| |
| // Admission control - verify preconditions for spinning |
| // |
| // We always spin a little bit, just to prevent _SpinDuration == 0 from |
| // becoming an absorbing state. Put another way, we spin briefly to |
| // sample, just in case the system load, parallelism, contention, or lock |
| // modality changed. |
| // |
| // Consider the following alternative: |
| // Periodically set _SpinDuration = _SpinLimit and try a long/full |
| // spin attempt. "Periodically" might mean after a tally of |
| // the # of failed spin attempts (or iterations) reaches some threshold. |
| // This takes us into the realm of 1-out-of-N spinning, where we |
| // hold the duration constant but vary the frequency. |
| |
| ctr = _SpinDuration ; |
| if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ; |
| if (ctr <= 0) return 0 ; |
| |
| if (Knob_SuccRestrict && _succ != NULL) return 0 ; |
| if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) { |
| TEVENT (Spin abort - notrunnable [TOP]); |
| return 0 ; |
| } |
| |
| int MaxSpin = Knob_MaxSpinners ; |
| if (MaxSpin >= 0) { |
| if (_Spinner > MaxSpin) { |
| TEVENT (Spin abort -- too many spinners) ; |
| return 0 ; |
| } |
| // Slighty racy, but benign ... |
| Adjust (&_Spinner, 1) ; |
| } |
| |
| // We're good to spin ... spin ingress. |
| // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades |
| // when preparing to LD...CAS _owner, etc and the CAS is likely |
| // to succeed. |
| int hits = 0 ; |
| int msk = 0 ; |
| int caspty = Knob_CASPenalty ; |
| int oxpty = Knob_OXPenalty ; |
| int sss = Knob_SpinSetSucc ; |
| if (sss && _succ == NULL ) _succ = Self ; |
| Thread * prv = NULL ; |
| |
| // There are three ways to exit the following loop: |
| // 1. A successful spin where this thread has acquired the lock. |
| // 2. Spin failure with prejudice |
| // 3. Spin failure without prejudice |
| |
| while (--ctr >= 0) { |
| |
| // Periodic polling -- Check for pending GC |
| // Threads may spin while they're unsafe. |
| // We don't want spinning threads to delay the JVM from reaching |
| // a stop-the-world safepoint or to steal cycles from GC. |
| // If we detect a pending safepoint we abort in order that |
| // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) |
| // this thread, if safe, doesn't steal cycles from GC. |
| // This is in keeping with the "no loitering in runtime" rule. |
| // We periodically check to see if there's a safepoint pending. |
| if ((ctr & 0xFF) == 0) { |
| if (SafepointSynchronize::do_call_back()) { |
| TEVENT (Spin: safepoint) ; |
| goto Abort ; // abrupt spin egress |
| } |
| if (Knob_UsePause & 1) SpinPause () ; |
| |
| int (*scb)(intptr_t,int) = SpinCallbackFunction ; |
| if (hits > 50 && scb != NULL) { |
| int abend = (*scb)(SpinCallbackArgument, 0) ; |
| } |
| } |
| |
| if (Knob_UsePause & 2) SpinPause() ; |
| |
| // Exponential back-off ... Stay off the bus to reduce coherency traffic. |
| // This is useful on classic SMP systems, but is of less utility on |
| // N1-style CMT platforms. |
| // |
| // Trade-off: lock acquisition latency vs coherency bandwidth. |
| // Lock hold times are typically short. A histogram |
| // of successful spin attempts shows that we usually acquire |
| // the lock early in the spin. That suggests we want to |
| // sample _owner frequently in the early phase of the spin, |
| // but then back-off and sample less frequently as the spin |
| // progresses. The back-off makes a good citizen on SMP big |
| // SMP systems. Oversampling _owner can consume excessive |
| // coherency bandwidth. Relatedly, if we _oversample _owner we |
| // can inadvertently interfere with the the ST m->owner=null. |
| // executed by the lock owner. |
| if (ctr & msk) continue ; |
| ++hits ; |
| if ((hits & 0xF) == 0) { |
| // The 0xF, above, corresponds to the exponent. |
| // Consider: (msk+1)|msk |
| msk = ((msk << 2)|3) & BackOffMask ; |
| } |
| |
| // Probe _owner with TATAS |
| // If this thread observes the monitor transition or flicker |
| // from locked to unlocked to locked, then the odds that this |
| // thread will acquire the lock in this spin attempt go down |
| // considerably. The same argument applies if the CAS fails |
| // or if we observe _owner change from one non-null value to |
| // another non-null value. In such cases we might abort |
| // the spin without prejudice or apply a "penalty" to the |
| // spin count-down variable "ctr", reducing it by 100, say. |
| |
| Thread * ox = (Thread *) _owner ; |
| if (ox == NULL) { |
| ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; |
| if (ox == NULL) { |
| // The CAS succeeded -- this thread acquired ownership |
| // Take care of some bookkeeping to exit spin state. |
| if (sss && _succ == Self) { |
| _succ = NULL ; |
| } |
| if (MaxSpin > 0) Adjust (&_Spinner, -1) ; |
| |
| // Increase _SpinDuration : |
| // The spin was successful (profitable) so we tend toward |
| // longer spin attempts in the future. |
| // CONSIDER: factor "ctr" into the _SpinDuration adjustment. |
| // If we acquired the lock early in the spin cycle it |
| // makes sense to increase _SpinDuration proportionally. |
| // Note that we don't clamp SpinDuration precisely at SpinLimit. |
| int x = _SpinDuration ; |
| if (x < Knob_SpinLimit) { |
| if (x < Knob_Poverty) x = Knob_Poverty ; |
| _SpinDuration = x + Knob_Bonus ; |
| } |
| return 1 ; |
| } |
| |
| // The CAS failed ... we can take any of the following actions: |
| // * penalize: ctr -= Knob_CASPenalty |
| // * exit spin with prejudice -- goto Abort; |
| // * exit spin without prejudice. |
| // * Since CAS is high-latency, retry again immediately. |
| prv = ox ; |
| TEVENT (Spin: cas failed) ; |
| if (caspty == -2) break ; |
| if (caspty == -1) goto Abort ; |
| ctr -= caspty ; |
| continue ; |
| } |
| |
| // Did lock ownership change hands ? |
| if (ox != prv && prv != NULL ) { |
| TEVENT (spin: Owner changed) |
| if (oxpty == -2) break ; |
| if (oxpty == -1) goto Abort ; |
| ctr -= oxpty ; |
| } |
| prv = ox ; |
| |
| // Abort the spin if the owner is not executing. |
| // The owner must be executing in order to drop the lock. |
| // Spinning while the owner is OFFPROC is idiocy. |
| // Consider: ctr -= RunnablePenalty ; |
| if (Knob_OState && NotRunnable (Self, ox)) { |
| TEVENT (Spin abort - notrunnable); |
| goto Abort ; |
| } |
| if (sss && _succ == NULL ) _succ = Self ; |
| } |
| |
| // Spin failed with prejudice -- reduce _SpinDuration. |
| // TODO: Use an AIMD-like policy to adjust _SpinDuration. |
| // AIMD is globally stable. |
| TEVENT (Spin failure) ; |
| { |
| int x = _SpinDuration ; |
| if (x > 0) { |
| // Consider an AIMD scheme like: x -= (x >> 3) + 100 |
| // This is globally sample and tends to damp the response. |
| x -= Knob_Penalty ; |
| if (x < 0) x = 0 ; |
| _SpinDuration = x ; |
| } |
| } |
| |
| Abort: |
| if (MaxSpin >= 0) Adjust (&_Spinner, -1) ; |
| if (sss && _succ == Self) { |
| _succ = NULL ; |
| // Invariant: after setting succ=null a contending thread |
| // must recheck-retry _owner before parking. This usually happens |
| // in the normal usage of TrySpin(), but it's safest |
| // to make TrySpin() as foolproof as possible. |
| OrderAccess::fence() ; |
| if (TryLock(Self) > 0) return 1 ; |
| } |
| return 0 ; |
| } |
| |
| // NotRunnable() -- informed spinning |
| // |
| // Don't bother spinning if the owner is not eligible to drop the lock. |
| // Peek at the owner's schedctl.sc_state and Thread._thread_values and |
| // spin only if the owner thread is _thread_in_Java or _thread_in_vm. |
| // The thread must be runnable in order to drop the lock in timely fashion. |
| // If the _owner is not runnable then spinning will not likely be |
| // successful (profitable). |
| // |
| // Beware -- the thread referenced by _owner could have died |
| // so a simply fetch from _owner->_thread_state might trap. |
| // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. |
| // Because of the lifecycle issues the schedctl and _thread_state values |
| // observed by NotRunnable() might be garbage. NotRunnable must |
| // tolerate this and consider the observed _thread_state value |
| // as advisory. |
| // |
| // Beware too, that _owner is sometimes a BasicLock address and sometimes |
| // a thread pointer. We differentiate the two cases with OwnerIsThread. |
| // Alternately, we might tag the type (thread pointer vs basiclock pointer) |
| // with the LSB of _owner. Another option would be to probablistically probe |
| // the putative _owner->TypeTag value. |
| // |
| // Checking _thread_state isn't perfect. Even if the thread is |
| // in_java it might be blocked on a page-fault or have been preempted |
| // and sitting on a ready/dispatch queue. _thread state in conjunction |
| // with schedctl.sc_state gives us a good picture of what the |
| // thread is doing, however. |
| // |
| // TODO: check schedctl.sc_state. |
| // We'll need to use SafeFetch32() to read from the schedctl block. |
| // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/ |
| // |
| // The return value from NotRunnable() is *advisory* -- the |
| // result is based on sampling and is not necessarily coherent. |
| // The caller must tolerate false-negative and false-positive errors. |
| // Spinning, in general, is probabilistic anyway. |
| |
| |
| int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) { |
| // Check either OwnerIsThread or ox->TypeTag == 2BAD. |
| if (!OwnerIsThread) return 0 ; |
| |
| if (ox == NULL) return 0 ; |
| |
| // Avoid transitive spinning ... |
| // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. |
| // Immediately after T1 acquires L it's possible that T2, also |
| // spinning on L, will see L.Owner=T1 and T1._Stalled=L. |
| // This occurs transiently after T1 acquired L but before |
| // T1 managed to clear T1.Stalled. T2 does not need to abort |
| // its spin in this circumstance. |
| intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ; |
| |
| if (BlockedOn == 1) return 1 ; |
| if (BlockedOn != 0) { |
| return BlockedOn != intptr_t(this) && _owner == ox ; |
| } |
| |
| assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ; |
| int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ; |
| // consider also: jst != _thread_in_Java -- but that's overspecific. |
| return jst == _thread_blocked || jst == _thread_in_native ; |
| } |
| |
| |
| // ----------------------------------------------------------------------------- |
| // WaitSet management ... |
| |
| ObjectWaiter::ObjectWaiter(Thread* thread) { |
| _next = NULL; |
| _prev = NULL; |
| _notified = 0; |
| TState = TS_RUN ; |
| _thread = thread; |
| _event = thread->_ParkEvent ; |
| _active = false; |
| assert (_event != NULL, "invariant") ; |
| } |
| |
| void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) { |
| JavaThread *jt = (JavaThread *)this->_thread; |
| _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); |
| } |
| |
| void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) { |
| JavaThread *jt = (JavaThread *)this->_thread; |
| JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); |
| } |
| |
| inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { |
| assert(node != NULL, "should not dequeue NULL node"); |
| assert(node->_prev == NULL, "node already in list"); |
| assert(node->_next == NULL, "node already in list"); |
| // put node at end of queue (circular doubly linked list) |
| if (_WaitSet == NULL) { |
| _WaitSet = node; |
| node->_prev = node; |
| node->_next = node; |
| } else { |
| ObjectWaiter* head = _WaitSet ; |
| ObjectWaiter* tail = head->_prev; |
| assert(tail->_next == head, "invariant check"); |
| tail->_next = node; |
| head->_prev = node; |
| node->_next = head; |
| node->_prev = tail; |
| } |
| } |
| |
| inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { |
| // dequeue the very first waiter |
| ObjectWaiter* waiter = _WaitSet; |
| if (waiter) { |
| DequeueSpecificWaiter(waiter); |
| } |
| return waiter; |
| } |
| |
| inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { |
| assert(node != NULL, "should not dequeue NULL node"); |
| assert(node->_prev != NULL, "node already removed from list"); |
| assert(node->_next != NULL, "node already removed from list"); |
| // when the waiter has woken up because of interrupt, |
| // timeout or other spurious wake-up, dequeue the |
| // waiter from waiting list |
| ObjectWaiter* next = node->_next; |
| if (next == node) { |
| assert(node->_prev == node, "invariant check"); |
| _WaitSet = NULL; |
| } else { |
| ObjectWaiter* prev = node->_prev; |
| assert(prev->_next == node, "invariant check"); |
| assert(next->_prev == node, "invariant check"); |
| next->_prev = prev; |
| prev->_next = next; |
| if (_WaitSet == node) { |
| _WaitSet = next; |
| } |
| } |
| node->_next = NULL; |
| node->_prev = NULL; |
| } |
| |
| // ----------------------------------------------------------------------------- |
| // PerfData support |
| PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ; |
| PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ; |
| PerfCounter * ObjectMonitor::_sync_Parks = NULL ; |
| PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ; |
| PerfCounter * ObjectMonitor::_sync_Notifications = NULL ; |
| PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ; |
| PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ; |
| PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ; |
| PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ; |
| PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ; |
| PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ; |
| PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ; |
| PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ; |
| PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ; |
| PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ; |
| PerfCounter * ObjectMonitor::_sync_Inflations = NULL ; |
| PerfCounter * ObjectMonitor::_sync_Deflations = NULL ; |
| PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ; |
| |
| // One-shot global initialization for the sync subsystem. |
| // We could also defer initialization and initialize on-demand |
| // the first time we call inflate(). Initialization would |
| // be protected - like so many things - by the MonitorCache_lock. |
| |
| void ObjectMonitor::Initialize () { |
| static int InitializationCompleted = 0 ; |
| assert (InitializationCompleted == 0, "invariant") ; |
| InitializationCompleted = 1 ; |
| if (UsePerfData) { |
| EXCEPTION_MARK ; |
| #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); } |
| #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); } |
| NEWPERFCOUNTER(_sync_Inflations) ; |
| NEWPERFCOUNTER(_sync_Deflations) ; |
| NEWPERFCOUNTER(_sync_ContendedLockAttempts) ; |
| NEWPERFCOUNTER(_sync_FutileWakeups) ; |
| NEWPERFCOUNTER(_sync_Parks) ; |
| NEWPERFCOUNTER(_sync_EmptyNotifications) ; |
| NEWPERFCOUNTER(_sync_Notifications) ; |
| NEWPERFCOUNTER(_sync_SlowEnter) ; |
| NEWPERFCOUNTER(_sync_SlowExit) ; |
| NEWPERFCOUNTER(_sync_SlowNotify) ; |
| NEWPERFCOUNTER(_sync_SlowNotifyAll) ; |
| NEWPERFCOUNTER(_sync_FailedSpins) ; |
| NEWPERFCOUNTER(_sync_SuccessfulSpins) ; |
| NEWPERFCOUNTER(_sync_PrivateA) ; |
| NEWPERFCOUNTER(_sync_PrivateB) ; |
| NEWPERFCOUNTER(_sync_MonInCirculation) ; |
| NEWPERFCOUNTER(_sync_MonScavenged) ; |
| NEWPERFVARIABLE(_sync_MonExtant) ; |
| #undef NEWPERFCOUNTER |
| } |
| } |
| |
| |
| // Compile-time asserts |
| // When possible, it's better to catch errors deterministically at |
| // compile-time than at runtime. The down-side to using compile-time |
| // asserts is that error message -- often something about negative array |
| // indices -- is opaque. |
| |
| #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); } |
| |
| void ObjectMonitor::ctAsserts() { |
| CTASSERT(offset_of (ObjectMonitor, _header) == 0); |
| } |
| |
| |
| static char * kvGet (char * kvList, const char * Key) { |
| if (kvList == NULL) return NULL ; |
| size_t n = strlen (Key) ; |
| char * Search ; |
| for (Search = kvList ; *Search ; Search += strlen(Search) + 1) { |
| if (strncmp (Search, Key, n) == 0) { |
| if (Search[n] == '=') return Search + n + 1 ; |
| if (Search[n] == 0) return (char *) "1" ; |
| } |
| } |
| return NULL ; |
| } |
| |
| static int kvGetInt (char * kvList, const char * Key, int Default) { |
| char * v = kvGet (kvList, Key) ; |
| int rslt = v ? ::strtol (v, NULL, 0) : Default ; |
| if (Knob_ReportSettings && v != NULL) { |
| ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ; |
| ::fflush (stdout) ; |
| } |
| return rslt ; |
| } |
| |
| void ObjectMonitor::DeferredInitialize () { |
| if (InitDone > 0) return ; |
| if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) { |
| while (InitDone != 1) ; |
| return ; |
| } |
| |
| // One-shot global initialization ... |
| // The initialization is idempotent, so we don't need locks. |
| // In the future consider doing this via os::init_2(). |
| // SyncKnobs consist of <Key>=<Value> pairs in the style |
| // of environment variables. Start by converting ':' to NUL. |
| |
| if (SyncKnobs == NULL) SyncKnobs = "" ; |
| |
| size_t sz = strlen (SyncKnobs) ; |
| char * knobs = (char *) malloc (sz + 2) ; |
| if (knobs == NULL) { |
| vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ; |
| guarantee (0, "invariant") ; |
| } |
| strcpy (knobs, SyncKnobs) ; |
| knobs[sz+1] = 0 ; |
| for (char * p = knobs ; *p ; p++) { |
| if (*p == ':') *p = 0 ; |
| } |
| |
| #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); } |
| SETKNOB(ReportSettings) ; |
| SETKNOB(Verbose) ; |
| SETKNOB(FixedSpin) ; |
| SETKNOB(SpinLimit) ; |
| SETKNOB(SpinBase) ; |
| SETKNOB(SpinBackOff); |
| SETKNOB(CASPenalty) ; |
| SETKNOB(OXPenalty) ; |
| SETKNOB(LogSpins) ; |
| SETKNOB(SpinSetSucc) ; |
| SETKNOB(SuccEnabled) ; |
| SETKNOB(SuccRestrict) ; |
| SETKNOB(Penalty) ; |
| SETKNOB(Bonus) ; |
| SETKNOB(BonusB) ; |
| SETKNOB(Poverty) ; |
| SETKNOB(SpinAfterFutile) ; |
| SETKNOB(UsePause) ; |
| SETKNOB(SpinEarly) ; |
| SETKNOB(OState) ; |
| SETKNOB(MaxSpinners) ; |
| SETKNOB(PreSpin) ; |
| SETKNOB(ExitPolicy) ; |
| SETKNOB(QMode); |
| SETKNOB(ResetEvent) ; |
| SETKNOB(MoveNotifyee) ; |
| SETKNOB(FastHSSEC) ; |
| #undef SETKNOB |
| |
| if (Knob_Verbose) { |
| sanity_checks(); |
| } |
| |
| if (os::is_MP()) { |
| BackOffMask = (1 << Knob_SpinBackOff) - 1 ; |
| if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ; |
| // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1) |
| } else { |
| Knob_SpinLimit = 0 ; |
| Knob_SpinBase = 0 ; |
| Knob_PreSpin = 0 ; |
| Knob_FixedSpin = -1 ; |
| } |
| |
| if (Knob_LogSpins == 0) { |
| ObjectMonitor::_sync_FailedSpins = NULL ; |
| } |
| |
| free (knobs) ; |
| OrderAccess::fence() ; |
| InitDone = 1 ; |
| } |
| |
| void ObjectMonitor::sanity_checks() { |
| int error_cnt = 0; |
| int warning_cnt = 0; |
| bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests); |
| |
| if (verbose) { |
| tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT, |
| sizeof(ObjectMonitor)); |
| } |
| |
| uint cache_line_size = VM_Version::L1_data_cache_line_size(); |
| if (verbose) { |
| tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size); |
| } |
| |
| ObjectMonitor dummy; |
| u_char *addr_begin = (u_char*)&dummy; |
| u_char *addr_header = (u_char*)&dummy._header; |
| u_char *addr_owner = (u_char*)&dummy._owner; |
| |
| uint offset_header = (uint)(addr_header - addr_begin); |
| if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header); |
| |
| uint offset_owner = (uint)(addr_owner - addr_begin); |
| if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner); |
| |
| if ((uint)(addr_header - addr_begin) != 0) { |
| tty->print_cr("ERROR: offset(_header) must be zero (0)."); |
| error_cnt++; |
| } |
| |
| if (cache_line_size != 0) { |
| // We were able to determine the L1 data cache line size so |
| // do some cache line specific sanity checks |
| |
| if ((offset_owner - offset_header) < cache_line_size) { |
| tty->print_cr("WARNING: the _header and _owner fields are closer " |
| "than a cache line which permits false sharing."); |
| warning_cnt++; |
| } |
| |
| if ((sizeof(ObjectMonitor) % cache_line_size) != 0) { |
| tty->print_cr("WARNING: ObjectMonitor size is not a multiple of " |
| "a cache line which permits false sharing."); |
| warning_cnt++; |
| } |
| } |
| |
| ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt, |
| &warning_cnt); |
| |
| if (verbose || error_cnt != 0 || warning_cnt != 0) { |
| tty->print_cr("INFO: error_cnt=%d", error_cnt); |
| tty->print_cr("INFO: warning_cnt=%d", warning_cnt); |
| } |
| |
| guarantee(error_cnt == 0, |
| "Fatal error(s) found in ObjectMonitor::sanity_checks()"); |
| } |
| |
| #ifndef PRODUCT |
| void ObjectMonitor::verify() { |
| } |
| |
| void ObjectMonitor::print() { |
| } |
| #endif |