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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
*
* 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 <cstring>
#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;
typedef uint32_t MonitorId;
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;
~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 both are enabled, we blindly 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_);
// 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_);
typedef std::list<Monitor*, TrackingAllocator<Monitor*, kAllocatorTagMonitorList>> Monitors;
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_
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