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* Copyright (C) 2008 The Android Open Source Project
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include <pthread.h>
#include <stdint.h>
#include <stdlib.h>
#include <iosfwd>
#include <list>
#include <vector>
#include "atomic.h"
#include "base/allocator.h"
#include "base/mutex.h"
#include "gc_root.h"
#include "lock_word.h"
#include "object_callbacks.h"
#include "read_barrier_option.h"
#include "thread_state.h"
namespace art {
class LockWord;
template<class T> class Handle;
class Thread;
class StackVisitor;
typedef uint32_t MonitorId;
namespace mirror {
class ArtMethod;
class Object;
} // namespace mirror
class Monitor {
// The default number of spins that are done before thread suspension is used to forcibly inflate
// a lock word. See Runtime::max_spins_before_thin_lock_inflation_.
constexpr static size_t kDefaultMaxSpinsBeforeThinLockInflation = 50;
static bool IsSensitiveThread();
static void Init(uint32_t lock_profiling_threshold, bool (*is_sensitive_thread_hook)());
// Return the thread id of the lock owner or 0 when there is no owner.
static uint32_t GetLockOwnerThreadId(mirror::Object* obj)
NO_THREAD_SAFETY_ANALYSIS; // TODO: Reading lock owner without holding lock is racy.
static mirror::Object* MonitorEnter(Thread* thread, mirror::Object* obj)
static bool MonitorExit(Thread* thread, mirror::Object* obj)
static void Notify(Thread* self, mirror::Object* obj)
SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
DoNotify(self, obj, false);
static void NotifyAll(Thread* self, mirror::Object* obj)
SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
DoNotify(self, obj, true);
// Object.wait(). Also called for class init.
static void Wait(Thread* self, mirror::Object* obj, int64_t ms, int32_t ns,
bool interruptShouldThrow, ThreadState why)
static void DescribeWait(std::ostream& os, const Thread* thread)
// Used to implement JDWP's ThreadReference.CurrentContendedMonitor.
static mirror::Object* GetContendedMonitor(Thread* thread)
// Calls 'callback' once for each lock held in the single stack frame represented by
// the current state of 'stack_visitor'.
// The abort_on_failure flag allows to not die when the state of the runtime is unorderly. This
// is necessary when we have already aborted but want to dump the stack as much as we can.
static void VisitLocks(StackVisitor* stack_visitor, void (*callback)(mirror::Object*, void*),
void* callback_context, bool abort_on_failure = true)
static bool IsValidLockWord(LockWord lock_word);
template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
mirror::Object* GetObject() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
return obj_.Read<kReadBarrierOption>();
void SetObject(mirror::Object* object);
Thread* GetOwner() const NO_THREAD_SAFETY_ANALYSIS {
return owner_;
int32_t GetHashCode();
bool IsLocked() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
bool HasHashCode() const {
return hash_code_.LoadRelaxed() != 0;
MonitorId GetMonitorId() const {
return monitor_id_;
// Inflate the lock on obj. May fail to inflate for spurious reasons, always re-check.
static void InflateThinLocked(Thread* self, Handle<mirror::Object> obj, LockWord lock_word,
uint32_t hash_code) NO_THREAD_SAFETY_ANALYSIS;
static bool Deflate(Thread* self, mirror::Object* obj)
// Not exclusive because ImageWriter calls this during a Heap::VisitObjects() that
// does not allow a thread suspension in the middle. TODO: maybe make this exclusive.
#ifndef __LP64__
void* operator new(size_t size) {
// Align Monitor* as per the monitor ID field size in the lock word.
void* result;
int error = posix_memalign(&result, LockWord::kMonitorIdAlignment, size);
CHECK_EQ(error, 0) << strerror(error);
return result;
void operator delete(void* ptr) {
explicit Monitor(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code)
explicit Monitor(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code,
MonitorId id) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
// Install the monitor into its object, may fail if another thread installs a different monitor
// first.
bool Install(Thread* self)
// Links a thread into a monitor's wait set. The monitor lock must be held by the caller of this
// routine.
void AppendToWaitSet(Thread* thread) EXCLUSIVE_LOCKS_REQUIRED(monitor_lock_);
// Unlinks a thread from a monitor's wait set. The monitor lock must be held by the caller of
// this routine.
void RemoveFromWaitSet(Thread* thread) EXCLUSIVE_LOCKS_REQUIRED(monitor_lock_);
// Changes the shape of a monitor from thin to fat, preserving the internal lock state. The
// calling thread must own the lock or the owner must be suspended. There's a race with other
// threads inflating the lock, installing hash codes and spurious failures. The caller should
// re-read the lock word following the call.
static void Inflate(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code)
void LogContentionEvent(Thread* self, uint32_t wait_ms, uint32_t sample_percent,
const char* owner_filename, uint32_t owner_line_number)
static void FailedUnlock(mirror::Object* obj, Thread* expected_owner, Thread* found_owner,
Monitor* mon)
void Lock(Thread* self)
bool Unlock(Thread* thread)
static void DoNotify(Thread* self, mirror::Object* obj, bool notify_all)
void Notify(Thread* self)
void NotifyAll(Thread* self)
// Wait on a monitor until timeout, interrupt, or notification. Used for Object.wait() and
// (somewhat indirectly) Thread.sleep() and Thread.join().
// If another thread calls Thread.interrupt(), we throw InterruptedException and return
// immediately if one of the following are true:
// - blocked in wait(), wait(long), or wait(long, int) methods of Object
// - blocked in join(), join(long), or join(long, int) methods of Thread
// - blocked in sleep(long), or sleep(long, int) methods of Thread
// Otherwise, we set the "interrupted" flag.
// Checks to make sure that "ns" is in the range 0-999999 (i.e. fractions of a millisecond) and
// throws the appropriate exception if it isn't.
// The spec allows "spurious wakeups", and recommends that all code using Object.wait() do so in
// a loop. This appears to derive from concerns about pthread_cond_wait() on multiprocessor
// systems. Some commentary on the web casts doubt on whether these can/should occur.
// Since we're allowed to wake up "early", we clamp extremely long durations to return at the end
// of the 32-bit time epoch.
void Wait(Thread* self, int64_t msec, int32_t nsec, bool interruptShouldThrow, ThreadState why)
// Translates the provided method and pc into its declaring class' source file and line number.
void TranslateLocation(mirror::ArtMethod* method, uint32_t pc,
const char** source_file, uint32_t* line_number) const
uint32_t GetOwnerThreadId();
static bool (*is_sensitive_thread_hook_)();
static uint32_t lock_profiling_threshold_;
ConditionVariable monitor_contenders_ GUARDED_BY(monitor_lock_);
// Number of people waiting on the condition.
size_t num_waiters_ GUARDED_BY(monitor_lock_);
// Which thread currently owns the lock?
Thread* volatile owner_ GUARDED_BY(monitor_lock_);
// Owner's recursive lock depth.
int lock_count_ GUARDED_BY(monitor_lock_);
// What object are we part of. This is a weak root. Do not access
// this directly, use GetObject() to read it so it will be guarded
// by a read barrier.
GcRoot<mirror::Object> obj_;
// Threads currently waiting on this monitor.
Thread* wait_set_ GUARDED_BY(monitor_lock_);
// Stored object hash code, generated lazily by GetHashCode.
AtomicInteger hash_code_;
// Method and dex pc where the lock owner acquired the lock, used when lock
// sampling is enabled. locking_method_ may be null if the lock is currently
// unlocked, or if the lock is acquired by the system when the stack is empty.
mirror::ArtMethod* locking_method_ GUARDED_BY(monitor_lock_);
uint32_t locking_dex_pc_ GUARDED_BY(monitor_lock_);
// The denser encoded version of this monitor as stored in the lock word.
MonitorId monitor_id_;
#ifdef __LP64__
// Free list for monitor pool.
Monitor* next_free_ GUARDED_BY(Locks::allocated_monitor_ids_lock_);
friend class MonitorInfo;
friend class MonitorList;
friend class MonitorPool;
friend class mirror::Object;
class MonitorList {
void Add(Monitor* m) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
void SweepMonitorList(IsMarkedCallback* callback, void* arg)
LOCKS_EXCLUDED(monitor_list_lock_) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
void DisallowNewMonitors() LOCKS_EXCLUDED(monitor_list_lock_);
void AllowNewMonitors() LOCKS_EXCLUDED(monitor_list_lock_);
void EnsureNewMonitorsDisallowed() LOCKS_EXCLUDED(monitor_list_lock_);
// Returns how many monitors were deflated.
size_t DeflateMonitors() LOCKS_EXCLUDED(monitor_list_lock_)
typedef std::list<Monitor*, TrackingAllocator<Monitor*, kAllocatorTagMonitorList>> Monitors;
// During sweeping we may free an object and on a separate thread have an object created using
// the newly freed memory. That object may then have its lock-word inflated and a monitor created.
// If we allow new monitor registration during sweeping this monitor may be incorrectly freed as
// the object wasn't marked when sweeping began.
bool allow_new_monitors_ GUARDED_BY(monitor_list_lock_);
Mutex monitor_list_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;
ConditionVariable monitor_add_condition_ GUARDED_BY(monitor_list_lock_);
Monitors list_ GUARDED_BY(monitor_list_lock_);
friend class Monitor;
// Collects information about the current state of an object's monitor.
// This is very unsafe, and must only be called when all threads are suspended.
// For use only by the JDWP implementation.
class MonitorInfo {
explicit MonitorInfo(mirror::Object* o) EXCLUSIVE_LOCKS_REQUIRED(Locks::mutator_lock_);
Thread* owner_;
size_t entry_count_;
std::vector<Thread*> waiters_;
} // namespace art