runtime/monitor.h - SHIFTPHONES/android_art - Gitiles

 /*
  * 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
  *
  *      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_MONITOR_H_
 #define ART_RUNTIME_MONITOR_H_

 #include <pthread.h>
 #include <stdint.h>
 #include <stdlib.h>

 #include <atomic>
 #include <iosfwd>
 #include <list>
 #include <vector>

 #include "base/allocator.h"
 #include "base/atomic.h"
 #include "base/mutex.h"
 #include "gc_root.h"
 #include "lock_word.h"
 #include "obj_ptr.h"
 #include "read_barrier_option.h"
 #include "runtime_callbacks.h"
 #include "thread_state.h"

 namespace art {

 class ArtMethod;
 class IsMarkedVisitor;
 class LockWord;
 template<class T> class Handle;
 class StackVisitor;
 class Thread;
 using MonitorId = uint32_t;

 namespace mirror {
 class Object;
 }  // namespace mirror

 enum class LockReason {
   kForWait,
   kForLock,
 };

 class Monitor {
  public:
   // 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 constexpr int kDefaultMonitorTimeoutMs = 500;

   static constexpr int kMonitorTimeoutMinMs = 200;

   static constexpr int kMonitorTimeoutMaxMs = 1000;  // 1 second

   ~Monitor();

   static void Init(uint32_t lock_profiling_threshold, uint32_t stack_dump_lock_profiling_threshold);

   // Return the thread id of the lock owner or 0 when there is no owner.
   static uint32_t GetLockOwnerThreadId(ObjPtr<mirror::Object> obj)
       NO_THREAD_SAFETY_ANALYSIS;  // TODO: Reading lock owner without holding lock is racy.

   // NO_THREAD_SAFETY_ANALYSIS for mon->Lock.
   static ObjPtr<mirror::Object> MonitorEnter(Thread* thread,
                                              ObjPtr<mirror::Object> obj,
                                              bool trylock)
       EXCLUSIVE_LOCK_FUNCTION(obj.Ptr())
       NO_THREAD_SAFETY_ANALYSIS
       REQUIRES(!Roles::uninterruptible_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // NO_THREAD_SAFETY_ANALYSIS for mon->Unlock.
   static bool MonitorExit(Thread* thread, ObjPtr<mirror::Object> obj)
       NO_THREAD_SAFETY_ANALYSIS
       REQUIRES(!Roles::uninterruptible_)
       REQUIRES_SHARED(Locks::mutator_lock_)
       UNLOCK_FUNCTION(obj.Ptr());

   static void Notify(Thread* self, ObjPtr<mirror::Object> obj)
       REQUIRES_SHARED(Locks::mutator_lock_) {
     DoNotify(self, obj, false);
   }
   static void NotifyAll(Thread* self, ObjPtr<mirror::Object> obj)
       REQUIRES_SHARED(Locks::mutator_lock_) {
     DoNotify(self, obj, true);
   }

   // Object.wait().  Also called for class init.
   // NO_THREAD_SAFETY_ANALYSIS for mon->Wait.
   static void Wait(Thread* self,
                    ObjPtr<mirror::Object> obj,
                    int64_t ms,
                    int32_t ns,
                    bool interruptShouldThrow, ThreadState why)
       REQUIRES_SHARED(Locks::mutator_lock_) NO_THREAD_SAFETY_ANALYSIS;

   static ThreadState FetchState(const Thread* thread,
                                 /* out */ ObjPtr<mirror::Object>* monitor_object,
                                 /* out */ uint32_t* lock_owner_tid)
       REQUIRES(!Locks::thread_suspend_count_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // Used to implement JDWP's ThreadReference.CurrentContendedMonitor.
   static ObjPtr<mirror::Object> GetContendedMonitor(Thread* thread)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // 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)(ObjPtr<mirror::Object>, void*),
                          void* callback_context,
                          bool abort_on_failure = true)
       REQUIRES_SHARED(Locks::mutator_lock_);

   static bool IsValidLockWord(LockWord lock_word);

   template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
   ObjPtr<mirror::Object> GetObject() REQUIRES_SHARED(Locks::mutator_lock_);

   void SetObject(ObjPtr<mirror::Object> object);

   // Provides no memory ordering guarantees.
   Thread* GetOwner() const {
     return owner_.load(std::memory_order_relaxed);
   }

   int32_t GetHashCode();

   // Is the monitor currently locked? Debug only, provides no memory ordering guarantees.
   bool IsLocked() REQUIRES_SHARED(Locks::mutator_lock_) REQUIRES(!monitor_lock_);

   bool HasHashCode() const {
     return hash_code_.load(std::memory_order_relaxed) != 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) REQUIRES_SHARED(Locks::mutator_lock_);

   // 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.
   // NO_THREAD_SAFETY_ANALYSIS for monitor->monitor_lock_.
   static bool Deflate(Thread* self, ObjPtr<mirror::Object> obj)
       REQUIRES_SHARED(Locks::mutator_lock_) NO_THREAD_SAFETY_ANALYSIS;

 #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) {
     free(ptr);
   }
 #endif

  private:
   Monitor(Thread* self, Thread* owner, ObjPtr<mirror::Object> obj, int32_t hash_code)
       REQUIRES_SHARED(Locks::mutator_lock_);
   Monitor(Thread* self, Thread* owner, ObjPtr<mirror::Object> obj, int32_t hash_code, MonitorId id)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // Install the monitor into its object, may fail if another thread installs a different monitor
   // first. Monitor remains in the same logical state as before, i.e. held the same # of times.
   bool Install(Thread* self)
       REQUIRES(!monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // 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) REQUIRES(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) REQUIRES(monitor_lock_);

   // Release the monitor lock and signal a waiting thread that has been notified and now needs the
   // lock. Assumes the monitor lock is held exactly once, and the owner_ field has been reset to
   // null. Caller may be suspended (Wait) or runnable (MonitorExit).
   void SignalWaiterAndReleaseMonitorLock(Thread* self) RELEASE(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, ObjPtr<mirror::Object> obj, int32_t hash_code)
       REQUIRES_SHARED(Locks::mutator_lock_)
       NO_THREAD_SAFETY_ANALYSIS;  // For m->Install(self)

   void LogContentionEvent(Thread* self,
                           uint32_t wait_ms,
                           uint32_t sample_percent,
                           ArtMethod* owner_method,
                           uint32_t owner_dex_pc)
       REQUIRES_SHARED(Locks::mutator_lock_);

   static void FailedUnlock(ObjPtr<mirror::Object> obj,
                            uint32_t expected_owner_thread_id,
                            uint32_t found_owner_thread_id,
                            Monitor* mon)
       REQUIRES(!Locks::thread_list_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // Try to lock without blocking, returns true if we acquired the lock.
   // If spin is true, then we spin for a short period before failing.
   bool TryLock(Thread* self, bool spin = false)
       TRY_ACQUIRE(true, monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   template<LockReason reason = LockReason::kForLock>
   void Lock(Thread* self)
       ACQUIRE(monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   bool Unlock(Thread* thread)
       RELEASE(monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   static void DoNotify(Thread* self, ObjPtr<mirror::Object> obj, bool notify_all)
       REQUIRES_SHARED(Locks::mutator_lock_) NO_THREAD_SAFETY_ANALYSIS;  // For mon->Notify.

   void Notify(Thread* self)
       REQUIRES(monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   void NotifyAll(Thread* self)
       REQUIRES(monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   static std::string PrettyContentionInfo(const std::string& owner_name,
                                           pid_t owner_tid,
                                           ArtMethod* owners_method,
                                           uint32_t owners_dex_pc,
                                           size_t num_waiters)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // 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)
       REQUIRES(monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // Translates the provided method and pc into its declaring class' source file and line number.
   static void TranslateLocation(ArtMethod* method, uint32_t pc,
                                 const char** source_file,
                                 int32_t* line_number)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // Provides no memory ordering guarantees.
   uint32_t GetOwnerThreadId() REQUIRES(!monitor_lock_);

   // Set locking_method_ and locking_dex_pc_ corresponding to owner's current stack.
   // owner is either self or suspended.
   void SetLockingMethod(Thread* owner) REQUIRES(monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // The same, but without checking for a proxy method. Currently requires owner == self.
   void SetLockingMethodNoProxy(Thread* owner) REQUIRES(monitor_lock_)
       REQUIRES_SHARED(Locks::mutator_lock_);

   // Support for systrace output of monitor operations.
   ALWAYS_INLINE static void AtraceMonitorLock(Thread* self,
                                               ObjPtr<mirror::Object> obj,
                                               bool is_wait)
       REQUIRES_SHARED(Locks::mutator_lock_);
   static void AtraceMonitorLockImpl(Thread* self,
                                     ObjPtr<mirror::Object> obj,
                                     bool is_wait)
       REQUIRES_SHARED(Locks::mutator_lock_);
   ALWAYS_INLINE static void AtraceMonitorUnlock();

   static uint32_t lock_profiling_threshold_;
   static uint32_t stack_dump_lock_profiling_threshold_;
   static bool capture_method_eagerly_;

   // Holding the monitor N times is represented by holding monitor_lock_ N times.
   Mutex monitor_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;

   // Pretend to unlock monitor lock.
   void FakeUnlockMonitorLock() RELEASE(monitor_lock_) NO_THREAD_SAFETY_ANALYSIS {}

   // Number of threads either waiting on the condition or waiting on a contended
   // monitor acquisition. Prevents deflation.
   std::atomic<size_t> num_waiters_;

   // Which thread currently owns the lock? monitor_lock_ only keeps the tid.
   // Only set while holding monitor_lock_. Non-locking readers only use it to
   // compare to self or for debugging.
   std::atomic<Thread*> owner_;

   // Owner's recursive lock depth. Owner_ non-null, and lock_count_ == 0 ==> held once.
   unsigned int lock_count_ GUARDED_BY(monitor_lock_);

   // Owner's recursive lock depth is given by monitor_lock_.GetDepth().

   // 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_);

   // Threads that were waiting on this monitor, but are now contending on it.
   Thread* wake_set_ GUARDED_BY(monitor_lock_);

   // Stored object hash code, generated lazily by GetHashCode.
   AtomicInteger hash_code_;

   // Data structure used to remember the method and dex pc of a recent holder of the
   // lock. Used for tracing and contention reporting. Setting these is expensive, since it
   // involves a partial stack walk. We set them only as follows, to minimize the cost:
   // - If tracing is enabled, they are needed immediately when we first notice contention, so we
   //   set them unconditionally when a monitor is acquired.
   // - If contention reporting is enabled, we use the lock_owner_request_ field to have the
   //   contending thread request them. The current owner then sets them when releasing the monitor,
   //   making them available when the contending thread acquires the monitor.
   // - If tracing and contention reporting are enabled, we do both. This usually prevents us from
   //   switching between reporting the end and beginning of critical sections for contention logging
   //   when tracing is enabled.  We expect that tracing overhead is normally much higher than for
   //   contention logging, so the added cost should be small. It also minimizes glitches when
   //   enabling and disabling traces.
   // We're tolerant of missing information. E.g. when tracing is initially turned on, we may
   // not have the lock holder information if the holder acquired the lock with tracing off.
   //
   // We make this data unconditionally atomic; for contention logging all accesses are in fact
   // protected by the monitor, but for tracing, reads are not. Writes are always
   // protected by the monitor.
   //
   // The fields are always accessed without memory ordering. We store a checksum, and reread if
   // the checksum doesn't correspond to the values.  This results in values that are correct with
   // very high probability, but not certainty.
   //
   // If we need lock_owner information for a certain thread for contenion logging, we store its
   // tid in lock_owner_request_. To satisfy the request, we store lock_owner_tid_,
   // lock_owner_method_, and lock_owner_dex_pc_ and the corresponding checksum while holding the
   // monitor.
   //
   // At all times, either lock_owner_ is zero, the checksum is valid, or a thread is actively
   // in the process of establishing one of those states. Only one thread at a time can be actively
   // establishing such a state, since writes are protected by the monitor.
   std::atomic<Thread*> lock_owner_;  // *lock_owner_ may no longer exist!
   std::atomic<ArtMethod*> lock_owner_method_;
   std::atomic<uint32_t> lock_owner_dex_pc_;
   std::atomic<uintptr_t> lock_owner_sum_;

   // Request lock owner save method and dex_pc. Written asynchronously.
   std::atomic<Thread*> lock_owner_request_;

   // Compute method, dex pc, and tid "checksum".
   uintptr_t LockOwnerInfoChecksum(ArtMethod* m, uint32_t dex_pc, Thread* t);

   // Set owning method, dex pc, and tid. owner_ field is set and points to current thread.
   void SetLockOwnerInfo(ArtMethod* method, uint32_t dex_pc, Thread* t)
       REQUIRES(monitor_lock_);

   // Get owning method and dex pc for the given thread, if available.
   void GetLockOwnerInfo(/*out*/ArtMethod** method, /*out*/uint32_t* dex_pc, Thread* t);

   // Do the same, while holding the monitor. There are no concurrent updates.
   void GetLockOwnerInfoLocked(/*out*/ArtMethod** method, /*out*/uint32_t* dex_pc,
                               uint32_t thread_id)
       REQUIRES(monitor_lock_);

   // We never clear lock_owner method and dex pc. Since it often reflects
   // ownership when we last detected contention, it may be inconsistent with owner_
   // and not 100% reliable. For lock contention monitoring, in the absence of tracing,
   // there is a small risk that the current owner may finish before noticing the request,
   // or the information will be overwritten by another intervening request and monitor
   // release, so it's also not 100% reliable. But if we report information at all, it
   // should generally (modulo accidental checksum matches) pertain to to an acquisition of the
   // right monitor by the right thread, so it's extremely unlikely to be seriously misleading.
   // Since we track threads by a pointer to the Thread structure, there is a small chance we may
   // confuse threads allocated at the same exact address, if a contending thread dies before
   // we inquire about it.

   // Check for and act on a pending lock_owner_request_
   void CheckLockOwnerRequest(Thread* self)
       REQUIRES(monitor_lock_) REQUIRES_SHARED(Locks::mutator_lock_);

   void MaybeEnableTimeout() REQUIRES(Locks::mutator_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_);
 #endif

   friend class MonitorInfo;
   friend class MonitorList;
   friend class MonitorPool;
   friend class mirror::Object;
   DISALLOW_COPY_AND_ASSIGN(Monitor);
 };

 class MonitorList {
  public:
   MonitorList();
   ~MonitorList();

   void Add(Monitor* m) REQUIRES_SHARED(Locks::mutator_lock_) REQUIRES(!monitor_list_lock_);

   void SweepMonitorList(IsMarkedVisitor* visitor)
       REQUIRES(!monitor_list_lock_) REQUIRES_SHARED(Locks::mutator_lock_);
   void DisallowNewMonitors() REQUIRES(!monitor_list_lock_);
   void AllowNewMonitors() REQUIRES(!monitor_list_lock_);
   void BroadcastForNewMonitors() REQUIRES(!monitor_list_lock_);
   // Returns how many monitors were deflated.
   size_t DeflateMonitors() REQUIRES(!monitor_list_lock_) REQUIRES(Locks::mutator_lock_);
   size_t Size() REQUIRES(!monitor_list_lock_);

   using Monitors = std::list<Monitor*, TrackingAllocator<Monitor*, kAllocatorTagMonitorList>>;

  private:
   // 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;
   DISALLOW_COPY_AND_ASSIGN(MonitorList);
 };

 // 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 {
  public:
   MonitorInfo() : owner_(nullptr), entry_count_(0) {}
   MonitorInfo(const MonitorInfo&) = default;
   MonitorInfo& operator=(const MonitorInfo&) = default;
   explicit MonitorInfo(ObjPtr<mirror::Object> o) REQUIRES(Locks::mutator_lock_);

   Thread* owner_;
   size_t entry_count_;
   std::vector<Thread*> waiters_;
 };

 }  // namespace art

 #endif  // ART_RUNTIME_MONITOR_H_
	/*
	* 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
	*
	* 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_MONITOR_H_
	#define ART_RUNTIME_MONITOR_H_

	#include <pthread.h>
	#include <stdint.h>
	#include <stdlib.h>

	#include <atomic>
	#include <iosfwd>
	#include <list>
	#include <vector>

	#include "base/allocator.h"
	#include "base/atomic.h"
	#include "base/mutex.h"
	#include "gc_root.h"
	#include "lock_word.h"
	#include "obj_ptr.h"
	#include "read_barrier_option.h"
	#include "runtime_callbacks.h"
	#include "thread_state.h"

	namespace art {

	class ArtMethod;
	class IsMarkedVisitor;
	class LockWord;
	template<class T> class Handle;
	class StackVisitor;
	class Thread;
	using MonitorId = uint32_t;

	namespace mirror {
	class Object;
	} // namespace mirror

	enum class LockReason {
	kForWait,
	kForLock,
	};

	class Monitor {
	public:
	// 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 constexpr int kDefaultMonitorTimeoutMs = 500;

	static constexpr int kMonitorTimeoutMinMs = 200;

	static constexpr int kMonitorTimeoutMaxMs = 1000; // 1 second

	~Monitor();

	static void Init(uint32_t lock_profiling_threshold, uint32_t stack_dump_lock_profiling_threshold);

	// Return the thread id of the lock owner or 0 when there is no owner.
	static uint32_t GetLockOwnerThreadId(ObjPtr<mirror::Object> obj)
	NO_THREAD_SAFETY_ANALYSIS; // TODO: Reading lock owner without holding lock is racy.

	// NO_THREAD_SAFETY_ANALYSIS for mon->Lock.
	static ObjPtr<mirror::Object> MonitorEnter(Thread* thread,
	ObjPtr<mirror::Object> obj,
	bool trylock)
	EXCLUSIVE_LOCK_FUNCTION(obj.Ptr())
	NO_THREAD_SAFETY_ANALYSIS
	REQUIRES(!Roles::uninterruptible_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// NO_THREAD_SAFETY_ANALYSIS for mon->Unlock.
	static bool MonitorExit(Thread* thread, ObjPtr<mirror::Object> obj)
	NO_THREAD_SAFETY_ANALYSIS
	REQUIRES(!Roles::uninterruptible_)
	REQUIRES_SHARED(Locks::mutator_lock_)
	UNLOCK_FUNCTION(obj.Ptr());

	static void Notify(Thread* self, ObjPtr<mirror::Object> obj)
	REQUIRES_SHARED(Locks::mutator_lock_) {
	DoNotify(self, obj, false);
	}
	static void NotifyAll(Thread* self, ObjPtr<mirror::Object> obj)
	REQUIRES_SHARED(Locks::mutator_lock_) {
	DoNotify(self, obj, true);
	}

	// Object.wait(). Also called for class init.
	// NO_THREAD_SAFETY_ANALYSIS for mon->Wait.
	static void Wait(Thread* self,
	ObjPtr<mirror::Object> obj,
	int64_t ms,
	int32_t ns,
	bool interruptShouldThrow, ThreadState why)
	REQUIRES_SHARED(Locks::mutator_lock_) NO_THREAD_SAFETY_ANALYSIS;

	static ThreadState FetchState(const Thread* thread,
	/* out / ObjPtr<mirror::Object> monitor_object,
	/* out / uint32_t lock_owner_tid)
	REQUIRES(!Locks::thread_suspend_count_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// Used to implement JDWP's ThreadReference.CurrentContendedMonitor.
	static ObjPtr<mirror::Object> GetContendedMonitor(Thread* thread)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// 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)(ObjPtr<mirror::Object>, void),
	void* callback_context,
	bool abort_on_failure = true)
	REQUIRES_SHARED(Locks::mutator_lock_);

	static bool IsValidLockWord(LockWord lock_word);

	template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
	ObjPtr<mirror::Object> GetObject() REQUIRES_SHARED(Locks::mutator_lock_);

	void SetObject(ObjPtr<mirror::Object> object);

	// Provides no memory ordering guarantees.
	Thread* GetOwner() const {
	return owner_.load(std::memory_order_relaxed);
	}

	int32_t GetHashCode();

	// Is the monitor currently locked? Debug only, provides no memory ordering guarantees.
	bool IsLocked() REQUIRES_SHARED(Locks::mutator_lock_) REQUIRES(!monitor_lock_);

	bool HasHashCode() const {
	return hash_code_.load(std::memory_order_relaxed) != 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) REQUIRES_SHARED(Locks::mutator_lock_);

	// 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.
	// NO_THREAD_SAFETY_ANALYSIS for monitor->monitor_lock_.
	static bool Deflate(Thread* self, ObjPtr<mirror::Object> obj)
	REQUIRES_SHARED(Locks::mutator_lock_) NO_THREAD_SAFETY_ANALYSIS;

	#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) {
	free(ptr);
	}
	#endif

	private:
	Monitor(Thread* self, Thread* owner, ObjPtr<mirror::Object> obj, int32_t hash_code)
	REQUIRES_SHARED(Locks::mutator_lock_);
	Monitor(Thread* self, Thread* owner, ObjPtr<mirror::Object> obj, int32_t hash_code, MonitorId id)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// Install the monitor into its object, may fail if another thread installs a different monitor
	// first. Monitor remains in the same logical state as before, i.e. held the same # of times.
	bool Install(Thread* self)
	REQUIRES(!monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// 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) REQUIRES(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) REQUIRES(monitor_lock_);

	// Release the monitor lock and signal a waiting thread that has been notified and now needs the
	// lock. Assumes the monitor lock is held exactly once, and the owner_ field has been reset to
	// null. Caller may be suspended (Wait) or runnable (MonitorExit).
	void SignalWaiterAndReleaseMonitorLock(Thread* self) RELEASE(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, ObjPtr<mirror::Object> obj, int32_t hash_code)
	REQUIRES_SHARED(Locks::mutator_lock_)
	NO_THREAD_SAFETY_ANALYSIS; // For m->Install(self)

	void LogContentionEvent(Thread* self,
	uint32_t wait_ms,
	uint32_t sample_percent,
	ArtMethod* owner_method,
	uint32_t owner_dex_pc)
	REQUIRES_SHARED(Locks::mutator_lock_);

	static void FailedUnlock(ObjPtr<mirror::Object> obj,
	uint32_t expected_owner_thread_id,
	uint32_t found_owner_thread_id,
	Monitor* mon)
	REQUIRES(!Locks::thread_list_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// Try to lock without blocking, returns true if we acquired the lock.
	// If spin is true, then we spin for a short period before failing.
	bool TryLock(Thread* self, bool spin = false)
	TRY_ACQUIRE(true, monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	template<LockReason reason = LockReason::kForLock>
	void Lock(Thread* self)
	ACQUIRE(monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	bool Unlock(Thread* thread)
	RELEASE(monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	static void DoNotify(Thread* self, ObjPtr<mirror::Object> obj, bool notify_all)
	REQUIRES_SHARED(Locks::mutator_lock_) NO_THREAD_SAFETY_ANALYSIS; // For mon->Notify.

	void Notify(Thread* self)
	REQUIRES(monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	void NotifyAll(Thread* self)
	REQUIRES(monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	static std::string PrettyContentionInfo(const std::string& owner_name,
	pid_t owner_tid,
	ArtMethod* owners_method,
	uint32_t owners_dex_pc,
	size_t num_waiters)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// 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)
	REQUIRES(monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// Translates the provided method and pc into its declaring class' source file and line number.
	static void TranslateLocation(ArtMethod* method, uint32_t pc,
	const char** source_file,
	int32_t* line_number)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// Provides no memory ordering guarantees.
	uint32_t GetOwnerThreadId() REQUIRES(!monitor_lock_);

	// Set locking_method_ and locking_dex_pc_ corresponding to owner's current stack.
	// owner is either self or suspended.
	void SetLockingMethod(Thread* owner) REQUIRES(monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// The same, but without checking for a proxy method. Currently requires owner == self.
	void SetLockingMethodNoProxy(Thread* owner) REQUIRES(monitor_lock_)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// Support for systrace output of monitor operations.
	ALWAYS_INLINE static void AtraceMonitorLock(Thread* self,
	ObjPtr<mirror::Object> obj,
	bool is_wait)
	REQUIRES_SHARED(Locks::mutator_lock_);
	static void AtraceMonitorLockImpl(Thread* self,
	ObjPtr<mirror::Object> obj,
	bool is_wait)
	REQUIRES_SHARED(Locks::mutator_lock_);
	ALWAYS_INLINE static void AtraceMonitorUnlock();

	static uint32_t lock_profiling_threshold_;
	static uint32_t stack_dump_lock_profiling_threshold_;
	static bool capture_method_eagerly_;

	// Holding the monitor N times is represented by holding monitor_lock_ N times.
	Mutex monitor_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;

	// Pretend to unlock monitor lock.
	void FakeUnlockMonitorLock() RELEASE(monitor_lock_) NO_THREAD_SAFETY_ANALYSIS {}

	// Number of threads either waiting on the condition or waiting on a contended
	// monitor acquisition. Prevents deflation.
	std::atomic<size_t> num_waiters_;

	// Which thread currently owns the lock? monitor_lock_ only keeps the tid.
	// Only set while holding monitor_lock_. Non-locking readers only use it to
	// compare to self or for debugging.
	std::atomic<Thread*> owner_;

	// Owner's recursive lock depth. Owner_ non-null, and lock_count_ == 0 ==> held once.
	unsigned int lock_count_ GUARDED_BY(monitor_lock_);

	// Owner's recursive lock depth is given by monitor_lock_.GetDepth().

	// 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_);

	// Threads that were waiting on this monitor, but are now contending on it.
	Thread* wake_set_ GUARDED_BY(monitor_lock_);

	// Stored object hash code, generated lazily by GetHashCode.
	AtomicInteger hash_code_;

	// Data structure used to remember the method and dex pc of a recent holder of the
	// lock. Used for tracing and contention reporting. Setting these is expensive, since it
	// involves a partial stack walk. We set them only as follows, to minimize the cost:
	// - If tracing is enabled, they are needed immediately when we first notice contention, so we
	// set them unconditionally when a monitor is acquired.
	// - If contention reporting is enabled, we use the lock_owner_request_ field to have the
	// contending thread request them. The current owner then sets them when releasing the monitor,
	// making them available when the contending thread acquires the monitor.
	// - If tracing and contention reporting are enabled, we do both. This usually prevents us from
	// switching between reporting the end and beginning of critical sections for contention logging
	// when tracing is enabled. We expect that tracing overhead is normally much higher than for
	// contention logging, so the added cost should be small. It also minimizes glitches when
	// enabling and disabling traces.
	// We're tolerant of missing information. E.g. when tracing is initially turned on, we may
	// not have the lock holder information if the holder acquired the lock with tracing off.
	//
	// We make this data unconditionally atomic; for contention logging all accesses are in fact
	// protected by the monitor, but for tracing, reads are not. Writes are always
	// protected by the monitor.
	//
	// The fields are always accessed without memory ordering. We store a checksum, and reread if
	// the checksum doesn't correspond to the values. This results in values that are correct with
	// very high probability, but not certainty.
	//
	// If we need lock_owner information for a certain thread for contenion logging, we store its
	// tid in lock_owner_request_. To satisfy the request, we store lock_owner_tid_,
	// lock_owner_method_, and lock_owner_dex_pc_ and the corresponding checksum while holding the
	// monitor.
	//
	// At all times, either lock_owner_ is zero, the checksum is valid, or a thread is actively
	// in the process of establishing one of those states. Only one thread at a time can be actively
	// establishing such a state, since writes are protected by the monitor.
	std::atomic<Thread> lock_owner_; // lock_owner_ may no longer exist!
	std::atomic<ArtMethod*> lock_owner_method_;
	std::atomic<uint32_t> lock_owner_dex_pc_;
	std::atomic<uintptr_t> lock_owner_sum_;

	// Request lock owner save method and dex_pc. Written asynchronously.
	std::atomic<Thread*> lock_owner_request_;

	// Compute method, dex pc, and tid "checksum".
	uintptr_t LockOwnerInfoChecksum(ArtMethod* m, uint32_t dex_pc, Thread* t);

	// Set owning method, dex pc, and tid. owner_ field is set and points to current thread.
	void SetLockOwnerInfo(ArtMethod* method, uint32_t dex_pc, Thread* t)
	REQUIRES(monitor_lock_);

	// Get owning method and dex pc for the given thread, if available.
	void GetLockOwnerInfo(/out/ArtMethod** method, /out/uint32_t* dex_pc, Thread* t);

	// Do the same, while holding the monitor. There are no concurrent updates.
	void GetLockOwnerInfoLocked(/out/ArtMethod** method, /out/uint32_t* dex_pc,
	uint32_t thread_id)
	REQUIRES(monitor_lock_);

	// We never clear lock_owner method and dex pc. Since it often reflects
	// ownership when we last detected contention, it may be inconsistent with owner_
	// and not 100% reliable. For lock contention monitoring, in the absence of tracing,
	// there is a small risk that the current owner may finish before noticing the request,
	// or the information will be overwritten by another intervening request and monitor
	// release, so it's also not 100% reliable. But if we report information at all, it
	// should generally (modulo accidental checksum matches) pertain to to an acquisition of the
	// right monitor by the right thread, so it's extremely unlikely to be seriously misleading.
	// Since we track threads by a pointer to the Thread structure, there is a small chance we may
	// confuse threads allocated at the same exact address, if a contending thread dies before
	// we inquire about it.

	// Check for and act on a pending lock_owner_request_
	void CheckLockOwnerRequest(Thread* self)
	REQUIRES(monitor_lock_) REQUIRES_SHARED(Locks::mutator_lock_);

	void MaybeEnableTimeout() REQUIRES(Locks::mutator_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_);
	#endif

	friend class MonitorInfo;
	friend class MonitorList;
	friend class MonitorPool;
	friend class mirror::Object;
	DISALLOW_COPY_AND_ASSIGN(Monitor);
	};

	class MonitorList {
	public:
	MonitorList();
	~MonitorList();

	void Add(Monitor* m) REQUIRES_SHARED(Locks::mutator_lock_) REQUIRES(!monitor_list_lock_);

	void SweepMonitorList(IsMarkedVisitor* visitor)
	REQUIRES(!monitor_list_lock_) REQUIRES_SHARED(Locks::mutator_lock_);
	void DisallowNewMonitors() REQUIRES(!monitor_list_lock_);
	void AllowNewMonitors() REQUIRES(!monitor_list_lock_);
	void BroadcastForNewMonitors() REQUIRES(!monitor_list_lock_);
	// Returns how many monitors were deflated.
	size_t DeflateMonitors() REQUIRES(!monitor_list_lock_) REQUIRES(Locks::mutator_lock_);
	size_t Size() REQUIRES(!monitor_list_lock_);

	using Monitors = std::list<Monitor, TrackingAllocator<Monitor, kAllocatorTagMonitorList>>;

	private:
	// 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;
	DISALLOW_COPY_AND_ASSIGN(MonitorList);
	};

	// 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 {
	public:
	MonitorInfo() : owner_(nullptr), entry_count_(0) {}
	MonitorInfo(const MonitorInfo&) = default;
	MonitorInfo& operator=(const MonitorInfo&) = default;
	explicit MonitorInfo(ObjPtr<mirror::Object> o) REQUIRES(Locks::mutator_lock_);

	Thread* owner_;
	size_t entry_count_;
	std::vector<Thread*> waiters_;
	};

	} // namespace art

	#endif // ART_RUNTIME_MONITOR_H_