runtime/thread_list.h - platform/art - Git at Google

 /*
  * Copyright (C) 2011 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
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #ifndef ART_RUNTIME_THREAD_LIST_H_
 #define ART_RUNTIME_THREAD_LIST_H_

 #include <bitset>
 #include <list>
 #include <vector>

 #include "barrier.h"
 #include "base/histogram.h"
 #include "base/mutex.h"
 #include "base/macros.h"
 #include "base/value_object.h"
 #include "jni.h"
 #include "reflective_handle_scope.h"
 #include "suspend_reason.h"
 #include "thread_state.h"

 namespace art HIDDEN {
 namespace gc {
 namespace collector {
 class GarbageCollector;
 }  // namespace collector
 class GcPauseListener;
 }  // namespace gc
 class Closure;
 class IsMarkedVisitor;
 class RootVisitor;
 class Thread;
 class TimingLogger;
 enum VisitRootFlags : uint8_t;

 class ThreadList {
  public:
   static constexpr uint32_t kMaxThreadId = 0xFFFF;
   static constexpr uint32_t kInvalidThreadId = 0;
   static constexpr uint32_t kMainThreadId = 1;
   static constexpr uint64_t kDefaultThreadSuspendTimeout =
       kIsDebugBuild ? 2'000'000'000ull : 4'000'000'000ull;
   // We fail more aggressively in debug builds to catch potential issues early.
   // The number of times we may retry when we find ourselves in a suspend-unfriendly state.
   static constexpr int kMaxSuspendRetries = kIsDebugBuild ? 500 : 5000;
   static constexpr useconds_t kThreadSuspendSleepUs = 100;

   explicit ThreadList(uint64_t thread_suspend_timeout_ns);
   ~ThreadList();

   void ShutDown();

   void DumpForSigQuit(std::ostream& os)
       REQUIRES(!Locks::thread_list_lock_, !Locks::mutator_lock_);
   // For thread suspend timeout dumps.
   EXPORT void Dump(std::ostream& os, bool dump_native_stack = true)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
   pid_t GetLockOwner();  // For SignalCatcher.

   // Thread suspension support.
   EXPORT void ResumeAll()
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_)
       UNLOCK_FUNCTION(Locks::mutator_lock_);
   EXPORT bool Resume(Thread* thread, SuspendReason reason = SuspendReason::kInternal)
       REQUIRES(!Locks::thread_suspend_count_lock_) WARN_UNUSED;

   // Suspends all other threads and gets exclusive access to the mutator lock.
   // If long_suspend is true, then other threads who try to suspend will never timeout.
   // long_suspend is currenly used for hprof since large heaps take a long time.
   EXPORT void SuspendAll(const char* cause, bool long_suspend = false)
       EXCLUSIVE_LOCK_FUNCTION(Locks::mutator_lock_)
       REQUIRES(!Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_,
                !Locks::mutator_lock_);

   // Suspend a thread using a peer, typically used by the debugger. Returns the thread on success,
   // else null. The peer is used to identify the thread to avoid races with the thread terminating.
   EXPORT Thread* SuspendThreadByPeer(jobject peer, SuspendReason reason)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);

   // Suspend a thread using its thread id, typically used by lock/monitor inflation. Returns the
   // thread on success else null. The thread id is used to identify the thread to avoid races with
   // the thread terminating. Note that as thread ids are recycled this may not suspend the expected
   // thread, that may be terminating. 'attempt_of_4' is zero if this is the only
   // attempt, or 1..4 to try 4 times with fractional timeouts.
   // TODO: Reconsider the use of thread_id, now that we have ThreadExitFlag.
   Thread* SuspendThreadByThreadId(uint32_t thread_id, SuspendReason reason, int attempt_of_4 = 0)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);

   // Find an existing thread (or self) by its thread id (not tid).
   EXPORT Thread* FindThreadByThreadId(uint32_t thread_id) REQUIRES(Locks::thread_list_lock_);

   // Find an existing thread (or self) by its tid (not thread id).
   Thread* FindThreadByTid(int tid) REQUIRES(Locks::thread_list_lock_);

   // Does the thread list still contain the given thread, or one at the same address?
   // Used by Monitor to provide (mostly accurate) debugging information.
   bool Contains(Thread* thread) REQUIRES(Locks::thread_list_lock_);

   // Run a checkpoint on all threads. Return the total number of threads for which the checkpoint
   // function has been or will be called.
   // Running threads are not suspended but run the checkpoint inside of the suspend check. The
   // return value includes already suspended threads for b/24191051. Runs or requests the
   // callback, if non-null, inside the thread_list_lock critical section after determining the
   // runnable/suspended states of the threads. Does not wait for completion of the checkpoint
   // function in running threads. If the caller holds the mutator lock, then all instances of the
   // checkpoint function are run with the mutator lock. If the caller does not hold the mutator
   // lock (see mutator_gc_coord.md) then, since the checkpoint code may not acquire or release the
   // mutator lock, the checkpoint will have no way to access Java data.
   // TODO: Is it possible to just require the mutator lock here?
   EXPORT size_t RunCheckpoint(Closure* checkpoint_function,
                        Closure* callback = nullptr,
                        bool allow_lock_checking = true)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

   // Convenience version of the above to disable lock checking inside Run function. Hopefully this
   // and the third parameter above will eventually disappear.
   size_t RunCheckpointUnchecked(Closure* checkpoint_function, Closure* callback = nullptr)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_) {
     return RunCheckpoint(checkpoint_function, callback, false);
   }

   // Run an empty checkpoint on threads. Wait until threads pass the next suspend point or are
   // suspended. This is used to ensure that the threads finish or aren't in the middle of an
   // in-flight mutator heap access (eg. a read barrier.) Runnable threads will respond by
   // decrementing the empty checkpoint barrier count. This works even when the weak ref access is
   // disabled. Only one concurrent use is currently supported.
   void RunEmptyCheckpoint()
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

   // Used to flip thread roots from from-space refs to to-space refs. Used only by the concurrent
   // moving collectors during a GC, and hence cannot be called from multiple threads concurrently.
   //
   // Briefly suspends all threads to atomically install a checkpoint-like thread_flip_visitor
   // function to be run on each thread. Run flip_callback while threads are suspended.
   // Thread_flip_visitors are run by each thread before it becomes runnable, or by us. We do not
   // return until all thread_flip_visitors have been run.
   void FlipThreadRoots(Closure* thread_flip_visitor,
                        Closure* flip_callback,
                        gc::collector::GarbageCollector* collector,
                        gc::GcPauseListener* pause_listener)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);

   // Iterates over all the threads.
   EXPORT void ForEach(void (*callback)(Thread*, void*), void* context)
       REQUIRES(Locks::thread_list_lock_);

   template<typename CallBack>
   void ForEach(CallBack cb) REQUIRES(Locks::thread_list_lock_) {
     ForEach([](Thread* t, void* ctx) REQUIRES(Locks::thread_list_lock_) {
       (*reinterpret_cast<CallBack*>(ctx))(t);
     }, &cb);
   }

   // Add/remove current thread from list.
   void Register(Thread* self)
       REQUIRES(Locks::runtime_shutdown_lock_)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);
   void Unregister(Thread* self, bool should_run_callbacks)
       REQUIRES(!Locks::mutator_lock_,
                !Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_);

   // Wait until there are no Unregister() requests in flight. Only makes sense when we know that
   // no new calls can be made. e.g. because we're the last thread.
   void WaitForUnregisterToComplete(Thread* self) REQUIRES(Locks::thread_list_lock_);

   void VisitRoots(RootVisitor* visitor, VisitRootFlags flags) const
       REQUIRES_SHARED(Locks::mutator_lock_);

   void VisitRootsForSuspendedThreads(RootVisitor* visitor)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   void VisitReflectiveTargets(ReflectiveValueVisitor* visitor) const REQUIRES(Locks::mutator_lock_);

   EXPORT void SweepInterpreterCaches(IsMarkedVisitor* visitor) const
       REQUIRES(Locks::mutator_lock_, !Locks::thread_list_lock_);

   // Return a copy of the thread list.
   std::list<Thread*> GetList() REQUIRES(Locks::thread_list_lock_) {
     return list_;
   }

   size_t Size() REQUIRES(Locks::thread_list_lock_) { return list_.size(); }

   void DumpNativeStacks(std::ostream& os)
       REQUIRES(!Locks::thread_list_lock_);

   Barrier* EmptyCheckpointBarrier() {
     return empty_checkpoint_barrier_.get();
   }

   void WaitForOtherNonDaemonThreadsToExit(bool check_no_birth = true)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_,
                !Locks::mutator_lock_);

   // Wait for suspend barrier to reach zero. Return a string possibly containing diagnostic
   // information on timeout, nothing on success.  The argument t specifies a thread to monitor for
   // the diagnostic information. If 0 is passed, we return an empty string on timeout.  Normally
   // the caller does not hold the mutator lock. See the comment at the call in
   // RequestSynchronousCheckpoint for the only exception.
   std::optional<std::string> WaitForSuspendBarrier(AtomicInteger* barrier,
                                                    pid_t t = 0,
                                                    int attempt_of_4 = 0)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

  private:
   uint32_t AllocThreadId(Thread* self);
   void ReleaseThreadId(Thread* self, uint32_t id) REQUIRES(!Locks::allocated_thread_ids_lock_);

   void DumpUnattachedThreads(std::ostream& os, bool dump_native_stack)
       REQUIRES(!Locks::thread_list_lock_);

   void SuspendAllDaemonThreadsForShutdown()
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

   void ResumeAllInternal(Thread* self)
       REQUIRES(Locks::thread_list_lock_, Locks::thread_suspend_count_lock_)
           UNLOCK_FUNCTION(Locks::mutator_lock_);

   // Helper to actually suspend a single thread. This is called with thread_list_lock_ held and
   // the caller guarantees that *thread is valid until that is released.  We "release the mutator
   // lock", by switching to self_state.  'attempt_of_4' is 0 if we only attempt once, and 1..4 if
   // we are going to try 4 times with a quarter of the full timeout. 'func_name' is used only to
   // identify ourselves for logging.
   bool SuspendThread(Thread* self,
                      Thread* thread,
                      SuspendReason reason,
                      ThreadState self_state,
                      const char* func_name,
                      int attempt_of_4) RELEASE(Locks::thread_list_lock_)
       RELEASE_SHARED(Locks::mutator_lock_);

   void SuspendAllInternal(Thread* self, SuspendReason reason = SuspendReason::kInternal)
       REQUIRES(!Locks::thread_list_lock_,
                !Locks::thread_suspend_count_lock_,
                !Locks::mutator_lock_);

   void AssertOtherThreadsAreSuspended(Thread* self)
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

   std::bitset<kMaxThreadId> allocated_ids_ GUARDED_BY(Locks::allocated_thread_ids_lock_);

   // The actual list of all threads.
   std::list<Thread*> list_ GUARDED_BY(Locks::thread_list_lock_);

   // Ongoing suspend all requests, used to ensure threads added to list_ respect SuspendAll, and
   // to ensure that only one SuspendAll ot FlipThreadRoots call is active at a time.  The value is
   // always either 0 or 1. Thread_suspend_count_lock must be held continuously while these two
   // functions modify suspend counts of all other threads and modify suspend_all_count_ .
   int suspend_all_count_ GUARDED_BY(Locks::thread_suspend_count_lock_);

   // Number of threads unregistering, ~ThreadList blocks until this hits 0.
   int unregistering_count_ GUARDED_BY(Locks::thread_list_lock_);

   // Thread suspend time histogram. Only modified when all the threads are suspended, so guarding
   // by mutator lock ensures no thread can read when another thread is modifying it.
   Histogram<uint64_t> suspend_all_histogram_ GUARDED_BY(Locks::mutator_lock_);

   // Whether or not the current thread suspension is long.
   bool long_suspend_;

   // Whether the shutdown function has been called. This is checked in the destructor. It is an
   // error to destroy a ThreadList instance without first calling ShutDown().
   bool shut_down_;

   // Thread suspension timeout in nanoseconds.
   const uint64_t thread_suspend_timeout_ns_;

   std::unique_ptr<Barrier> empty_checkpoint_barrier_;

   friend class Thread;

   friend class Mutex;
   friend class BaseMutex;

   DISALLOW_COPY_AND_ASSIGN(ThreadList);
 };

 // Helper for suspending all threads and getting exclusive access to the mutator lock.
 class ScopedSuspendAll : public ValueObject {
  public:
   EXPORT explicit ScopedSuspendAll(const char* cause, bool long_suspend = false)
      EXCLUSIVE_LOCK_FUNCTION(Locks::mutator_lock_)
      REQUIRES(!Locks::thread_list_lock_,
               !Locks::thread_suspend_count_lock_,
               !Locks::mutator_lock_);
   // No REQUIRES(mutator_lock_) since the unlock function already asserts this.
   EXPORT ~ScopedSuspendAll()
       REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_)
       UNLOCK_FUNCTION(Locks::mutator_lock_);
 };

 }  // namespace art

 #endif  // ART_RUNTIME_THREAD_LIST_H_
	/*
	* Copyright (C) 2011 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
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#ifndef ART_RUNTIME_THREAD_LIST_H_
	#define ART_RUNTIME_THREAD_LIST_H_

	#include <bitset>
	#include <list>
	#include <vector>

	#include "barrier.h"
	#include "base/histogram.h"
	#include "base/mutex.h"
	#include "base/macros.h"
	#include "base/value_object.h"
	#include "jni.h"
	#include "reflective_handle_scope.h"
	#include "suspend_reason.h"
	#include "thread_state.h"

	namespace art HIDDEN {
	namespace gc {
	namespace collector {
	class GarbageCollector;
	} // namespace collector
	class GcPauseListener;
	} // namespace gc
	class Closure;
	class IsMarkedVisitor;
	class RootVisitor;
	class Thread;
	class TimingLogger;
	enum VisitRootFlags : uint8_t;

	class ThreadList {
	public:
	static constexpr uint32_t kMaxThreadId = 0xFFFF;
	static constexpr uint32_t kInvalidThreadId = 0;
	static constexpr uint32_t kMainThreadId = 1;
	static constexpr uint64_t kDefaultThreadSuspendTimeout =
	kIsDebugBuild ? 2'000'000'000ull : 4'000'000'000ull;
	// We fail more aggressively in debug builds to catch potential issues early.
	// The number of times we may retry when we find ourselves in a suspend-unfriendly state.
	static constexpr int kMaxSuspendRetries = kIsDebugBuild ? 500 : 5000;
	static constexpr useconds_t kThreadSuspendSleepUs = 100;

	explicit ThreadList(uint64_t thread_suspend_timeout_ns);
	~ThreadList();

	void ShutDown();

	void DumpForSigQuit(std::ostream& os)
	REQUIRES(!Locks::thread_list_lock_, !Locks::mutator_lock_);
	// For thread suspend timeout dumps.
	EXPORT void Dump(std::ostream& os, bool dump_native_stack = true)
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);
	pid_t GetLockOwner(); // For SignalCatcher.

	// Thread suspension support.
	EXPORT void ResumeAll()
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_)
	UNLOCK_FUNCTION(Locks::mutator_lock_);
	EXPORT bool Resume(Thread* thread, SuspendReason reason = SuspendReason::kInternal)
	REQUIRES(!Locks::thread_suspend_count_lock_) WARN_UNUSED;

	// Suspends all other threads and gets exclusive access to the mutator lock.
	// If long_suspend is true, then other threads who try to suspend will never timeout.
	// long_suspend is currenly used for hprof since large heaps take a long time.
	EXPORT void SuspendAll(const char* cause, bool long_suspend = false)
	EXCLUSIVE_LOCK_FUNCTION(Locks::mutator_lock_)
	REQUIRES(!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_,
	!Locks::mutator_lock_);

	// Suspend a thread using a peer, typically used by the debugger. Returns the thread on success,
	// else null. The peer is used to identify the thread to avoid races with the thread terminating.
	EXPORT Thread* SuspendThreadByPeer(jobject peer, SuspendReason reason)
	REQUIRES(!Locks::mutator_lock_,
	!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_);

	// Suspend a thread using its thread id, typically used by lock/monitor inflation. Returns the
	// thread on success else null. The thread id is used to identify the thread to avoid races with
	// the thread terminating. Note that as thread ids are recycled this may not suspend the expected
	// thread, that may be terminating. 'attempt_of_4' is zero if this is the only
	// attempt, or 1..4 to try 4 times with fractional timeouts.
	// TODO: Reconsider the use of thread_id, now that we have ThreadExitFlag.
	Thread* SuspendThreadByThreadId(uint32_t thread_id, SuspendReason reason, int attempt_of_4 = 0)
	REQUIRES(!Locks::mutator_lock_,
	!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_);

	// Find an existing thread (or self) by its thread id (not tid).
	EXPORT Thread* FindThreadByThreadId(uint32_t thread_id) REQUIRES(Locks::thread_list_lock_);

	// Find an existing thread (or self) by its tid (not thread id).
	Thread* FindThreadByTid(int tid) REQUIRES(Locks::thread_list_lock_);

	// Does the thread list still contain the given thread, or one at the same address?
	// Used by Monitor to provide (mostly accurate) debugging information.
	bool Contains(Thread* thread) REQUIRES(Locks::thread_list_lock_);

	// Run a checkpoint on all threads. Return the total number of threads for which the checkpoint
	// function has been or will be called.
	// Running threads are not suspended but run the checkpoint inside of the suspend check. The
	// return value includes already suspended threads for b/24191051. Runs or requests the
	// callback, if non-null, inside the thread_list_lock critical section after determining the
	// runnable/suspended states of the threads. Does not wait for completion of the checkpoint
	// function in running threads. If the caller holds the mutator lock, then all instances of the
	// checkpoint function are run with the mutator lock. If the caller does not hold the mutator
	// lock (see mutator_gc_coord.md) then, since the checkpoint code may not acquire or release the
	// mutator lock, the checkpoint will have no way to access Java data.
	// TODO: Is it possible to just require the mutator lock here?
	EXPORT size_t RunCheckpoint(Closure* checkpoint_function,
	Closure* callback = nullptr,
	bool allow_lock_checking = true)
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

	// Convenience version of the above to disable lock checking inside Run function. Hopefully this
	// and the third parameter above will eventually disappear.
	size_t RunCheckpointUnchecked(Closure* checkpoint_function, Closure* callback = nullptr)
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_) {
	return RunCheckpoint(checkpoint_function, callback, false);
	}

	// Run an empty checkpoint on threads. Wait until threads pass the next suspend point or are
	// suspended. This is used to ensure that the threads finish or aren't in the middle of an
	// in-flight mutator heap access (eg. a read barrier.) Runnable threads will respond by
	// decrementing the empty checkpoint barrier count. This works even when the weak ref access is
	// disabled. Only one concurrent use is currently supported.
	void RunEmptyCheckpoint()
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

	// Used to flip thread roots from from-space refs to to-space refs. Used only by the concurrent
	// moving collectors during a GC, and hence cannot be called from multiple threads concurrently.
	//
	// Briefly suspends all threads to atomically install a checkpoint-like thread_flip_visitor
	// function to be run on each thread. Run flip_callback while threads are suspended.
	// Thread_flip_visitors are run by each thread before it becomes runnable, or by us. We do not
	// return until all thread_flip_visitors have been run.
	void FlipThreadRoots(Closure* thread_flip_visitor,
	Closure* flip_callback,
	gc::collector::GarbageCollector* collector,
	gc::GcPauseListener* pause_listener)
	REQUIRES(!Locks::mutator_lock_,
	!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_);

	// Iterates over all the threads.
	EXPORT void ForEach(void (callback)(Thread, void), void context)
	REQUIRES(Locks::thread_list_lock_);

	template<typename CallBack>
	void ForEach(CallBack cb) REQUIRES(Locks::thread_list_lock_) {
	ForEach([](Thread* t, void* ctx) REQUIRES(Locks::thread_list_lock_) {
	(reinterpret_cast<CallBack>(ctx))(t);
	}, &cb);
	}

	// Add/remove current thread from list.
	void Register(Thread* self)
	REQUIRES(Locks::runtime_shutdown_lock_)
	REQUIRES(!Locks::mutator_lock_,
	!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_);
	void Unregister(Thread* self, bool should_run_callbacks)
	REQUIRES(!Locks::mutator_lock_,
	!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_);

	// Wait until there are no Unregister() requests in flight. Only makes sense when we know that
	// no new calls can be made. e.g. because we're the last thread.
	void WaitForUnregisterToComplete(Thread* self) REQUIRES(Locks::thread_list_lock_);

	void VisitRoots(RootVisitor* visitor, VisitRootFlags flags) const
	REQUIRES_SHARED(Locks::mutator_lock_);

	void VisitRootsForSuspendedThreads(RootVisitor* visitor)
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	void VisitReflectiveTargets(ReflectiveValueVisitor* visitor) const REQUIRES(Locks::mutator_lock_);

	EXPORT void SweepInterpreterCaches(IsMarkedVisitor* visitor) const
	REQUIRES(Locks::mutator_lock_, !Locks::thread_list_lock_);

	// Return a copy of the thread list.
	std::list<Thread*> GetList() REQUIRES(Locks::thread_list_lock_) {
	return list_;
	}

	size_t Size() REQUIRES(Locks::thread_list_lock_) { return list_.size(); }

	void DumpNativeStacks(std::ostream& os)
	REQUIRES(!Locks::thread_list_lock_);

	Barrier* EmptyCheckpointBarrier() {
	return empty_checkpoint_barrier_.get();
	}

	void WaitForOtherNonDaemonThreadsToExit(bool check_no_birth = true)
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_,
	!Locks::mutator_lock_);

	// Wait for suspend barrier to reach zero. Return a string possibly containing diagnostic
	// information on timeout, nothing on success. The argument t specifies a thread to monitor for
	// the diagnostic information. If 0 is passed, we return an empty string on timeout. Normally
	// the caller does not hold the mutator lock. See the comment at the call in
	// RequestSynchronousCheckpoint for the only exception.
	std::optional<std::string> WaitForSuspendBarrier(AtomicInteger* barrier,
	pid_t t = 0,
	int attempt_of_4 = 0)
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

	private:
	uint32_t AllocThreadId(Thread* self);
	void ReleaseThreadId(Thread* self, uint32_t id) REQUIRES(!Locks::allocated_thread_ids_lock_);

	void DumpUnattachedThreads(std::ostream& os, bool dump_native_stack)
	REQUIRES(!Locks::thread_list_lock_);

	void SuspendAllDaemonThreadsForShutdown()
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

	void ResumeAllInternal(Thread* self)
	REQUIRES(Locks::thread_list_lock_, Locks::thread_suspend_count_lock_)
	UNLOCK_FUNCTION(Locks::mutator_lock_);

	// Helper to actually suspend a single thread. This is called with thread_list_lock_ held and
	// the caller guarantees that *thread is valid until that is released. We "release the mutator
	// lock", by switching to self_state. 'attempt_of_4' is 0 if we only attempt once, and 1..4 if
	// we are going to try 4 times with a quarter of the full timeout. 'func_name' is used only to
	// identify ourselves for logging.
	bool SuspendThread(Thread* self,
	Thread* thread,
	SuspendReason reason,
	ThreadState self_state,
	const char* func_name,
	int attempt_of_4) RELEASE(Locks::thread_list_lock_)
	RELEASE_SHARED(Locks::mutator_lock_);

	void SuspendAllInternal(Thread* self, SuspendReason reason = SuspendReason::kInternal)
	REQUIRES(!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_,
	!Locks::mutator_lock_);

	void AssertOtherThreadsAreSuspended(Thread* self)
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_);

	std::bitset<kMaxThreadId> allocated_ids_ GUARDED_BY(Locks::allocated_thread_ids_lock_);

	// The actual list of all threads.
	std::list<Thread*> list_ GUARDED_BY(Locks::thread_list_lock_);

	// Ongoing suspend all requests, used to ensure threads added to list_ respect SuspendAll, and
	// to ensure that only one SuspendAll ot FlipThreadRoots call is active at a time. The value is
	// always either 0 or 1. Thread_suspend_count_lock must be held continuously while these two
	// functions modify suspend counts of all other threads and modify suspend_all_count_ .
	int suspend_all_count_ GUARDED_BY(Locks::thread_suspend_count_lock_);

	// Number of threads unregistering, ~ThreadList blocks until this hits 0.
	int unregistering_count_ GUARDED_BY(Locks::thread_list_lock_);

	// Thread suspend time histogram. Only modified when all the threads are suspended, so guarding
	// by mutator lock ensures no thread can read when another thread is modifying it.
	Histogram<uint64_t> suspend_all_histogram_ GUARDED_BY(Locks::mutator_lock_);

	// Whether or not the current thread suspension is long.
	bool long_suspend_;

	// Whether the shutdown function has been called. This is checked in the destructor. It is an
	// error to destroy a ThreadList instance without first calling ShutDown().
	bool shut_down_;

	// Thread suspension timeout in nanoseconds.
	const uint64_t thread_suspend_timeout_ns_;

	std::unique_ptr<Barrier> empty_checkpoint_barrier_;

	friend class Thread;

	friend class Mutex;
	friend class BaseMutex;

	DISALLOW_COPY_AND_ASSIGN(ThreadList);
	};

	// Helper for suspending all threads and getting exclusive access to the mutator lock.
	class ScopedSuspendAll : public ValueObject {
	public:
	EXPORT explicit ScopedSuspendAll(const char* cause, bool long_suspend = false)
	EXCLUSIVE_LOCK_FUNCTION(Locks::mutator_lock_)
	REQUIRES(!Locks::thread_list_lock_,
	!Locks::thread_suspend_count_lock_,
	!Locks::mutator_lock_);
	// No REQUIRES(mutator_lock_) since the unlock function already asserts this.
	EXPORT ~ScopedSuspendAll()
	REQUIRES(!Locks::thread_list_lock_, !Locks::thread_suspend_count_lock_)
	UNLOCK_FUNCTION(Locks::mutator_lock_);
	};

	} // namespace art

	#endif // ART_RUNTIME_THREAD_LIST_H_