/*
* 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.
*/
#include "monitor.h"
#include <vector>
#include "base/mutex.h"
#include "base/stl_util.h"
#include "class_linker.h"
#include "dex_file-inl.h"
#include "dex_instruction.h"
#include "lock_word-inl.h"
#include "mirror/art_method-inl.h"
#include "mirror/class-inl.h"
#include "mirror/object-inl.h"
#include "mirror/object_array-inl.h"
#include "scoped_thread_state_change.h"
#include "thread.h"
#include "thread_list.h"
#include "verifier/method_verifier.h"
#include "well_known_classes.h"
namespace art {
static constexpr uint64_t kLongWaitMs = 100;
/*
* Every Object has a monitor associated with it, but not every Object is actually locked. Even
* the ones that are locked do not need a full-fledged monitor until a) there is actual contention
* or b) wait() is called on the Object.
*
* For Android, we have implemented a scheme similar to the one described in Bacon et al.'s
* "Thin locks: featherweight synchronization for Java" (ACM 1998). Things are even easier for us,
* though, because we have a full 32 bits to work with.
*
* The two states of an Object's lock are referred to as "thin" and "fat". A lock may transition
* from the "thin" state to the "fat" state and this transition is referred to as inflation. Once
* a lock has been inflated it remains in the "fat" state indefinitely.
*
* The lock value itself is stored in mirror::Object::monitor_ and the representation is described
* in the LockWord value type.
*
* Monitors provide:
* - mutually exclusive access to resources
* - a way for multiple threads to wait for notification
*
* In effect, they fill the role of both mutexes and condition variables.
*
* Only one thread can own the monitor at any time. There may be several threads waiting on it
* (the wait call unlocks it). One or more waiting threads may be getting interrupted or notified
* at any given time.
*/
bool (*Monitor::is_sensitive_thread_hook_)() = NULL;
uint32_t Monitor::lock_profiling_threshold_ = 0;
bool Monitor::IsSensitiveThread() {
if (is_sensitive_thread_hook_ != NULL) {
return (*is_sensitive_thread_hook_)();
}
return false;
}
void Monitor::Init(uint32_t lock_profiling_threshold, bool (*is_sensitive_thread_hook)()) {
lock_profiling_threshold_ = lock_profiling_threshold;
is_sensitive_thread_hook_ = is_sensitive_thread_hook;
}
Monitor::Monitor(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code)
: monitor_lock_("a monitor lock", kMonitorLock),
monitor_contenders_("monitor contenders", monitor_lock_),
num_waiters_(0),
owner_(owner),
lock_count_(0),
obj_(GcRoot<mirror::Object>(obj)),
wait_set_(NULL),
hash_code_(hash_code),
locking_method_(NULL),
locking_dex_pc_(0),
monitor_id_(MonitorPool::ComputeMonitorId(this, self)) {
#ifdef __LP64__
DCHECK(false) << "Should not be reached in 64b";
next_free_ = nullptr;
#endif
// We should only inflate a lock if the owner is ourselves or suspended. This avoids a race
// with the owner unlocking the thin-lock.
CHECK(owner == nullptr || owner == self || owner->IsSuspended());
// The identity hash code is set for the life time of the monitor.
}
Monitor::Monitor(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code,
MonitorId id)
: monitor_lock_("a monitor lock", kMonitorLock),
monitor_contenders_("monitor contenders", monitor_lock_),
num_waiters_(0),
owner_(owner),
lock_count_(0),
obj_(GcRoot<mirror::Object>(obj)),
wait_set_(NULL),
hash_code_(hash_code),
locking_method_(NULL),
locking_dex_pc_(0),
monitor_id_(id) {
#ifdef __LP64__
next_free_ = nullptr;
#endif
// We should only inflate a lock if the owner is ourselves or suspended. This avoids a race
// with the owner unlocking the thin-lock.
CHECK(owner == nullptr || owner == self || owner->IsSuspended());
// The identity hash code is set for the life time of the monitor.
}
int32_t Monitor::GetHashCode() {
while (!HasHashCode()) {
if (hash_code_.CompareExchangeWeakRelaxed(0, mirror::Object::GenerateIdentityHashCode())) {
break;
}
}
DCHECK(HasHashCode());
return hash_code_.LoadRelaxed();
}
bool Monitor::Install(Thread* self) {
MutexLock mu(self, monitor_lock_); // Uncontended mutex acquisition as monitor isn't yet public.
CHECK(owner_ == nullptr || owner_ == self || owner_->IsSuspended());
// Propagate the lock state.
LockWord lw(GetObject()->GetLockWord(false));
switch (lw.GetState()) {
case LockWord::kThinLocked: {
CHECK_EQ(owner_->GetThreadId(), lw.ThinLockOwner());
lock_count_ = lw.ThinLockCount();
break;
}
case LockWord::kHashCode: {
CHECK_EQ(hash_code_.LoadRelaxed(), static_cast<int32_t>(lw.GetHashCode()));
break;
}
case LockWord::kFatLocked: {
// The owner_ is suspended but another thread beat us to install a monitor.
return false;
}
case LockWord::kUnlocked: {
LOG(FATAL) << "Inflating unlocked lock word";
break;
}
default: {
LOG(FATAL) << "Invalid monitor state " << lw.GetState();
return false;
}
}
LockWord fat(this);
// Publish the updated lock word, which may race with other threads.
bool success = GetObject()->CasLockWordWeakSequentiallyConsistent(lw, fat);
// Lock profiling.
if (success && owner_ != nullptr && lock_profiling_threshold_ != 0) {
// Do not abort on dex pc errors. This can easily happen when we want to dump a stack trace on
// abort.
locking_method_ = owner_->GetCurrentMethod(&locking_dex_pc_, false);
}
return success;
}
Monitor::~Monitor() {
// Deflated monitors have a null object.
}
/*
* Links a thread into a monitor's wait set. The monitor lock must be
* held by the caller of this routine.
*/
void Monitor::AppendToWaitSet(Thread* thread) {
DCHECK(owner_ == Thread::Current());
DCHECK(thread != NULL);
DCHECK(thread->GetWaitNext() == nullptr) << thread->GetWaitNext();
if (wait_set_ == NULL) {
wait_set_ = thread;
return;
}
// push_back.
Thread* t = wait_set_;
while (t->GetWaitNext() != nullptr) {
t = t->GetWaitNext();
}
t->SetWaitNext(thread);
}
/*
* Unlinks a thread from a monitor's wait set. The monitor lock must
* be held by the caller of this routine.
*/
void Monitor::RemoveFromWaitSet(Thread *thread) {
DCHECK(owner_ == Thread::Current());
DCHECK(thread != NULL);
if (wait_set_ == NULL) {
return;
}
if (wait_set_ == thread) {
wait_set_ = thread->GetWaitNext();
thread->SetWaitNext(nullptr);
return;
}
Thread* t = wait_set_;
while (t->GetWaitNext() != NULL) {
if (t->GetWaitNext() == thread) {
t->SetWaitNext(thread->GetWaitNext());
thread->SetWaitNext(nullptr);
return;
}
t = t->GetWaitNext();
}
}
void Monitor::SetObject(mirror::Object* object) {
obj_ = GcRoot<mirror::Object>(object);
}
void Monitor::Lock(Thread* self) {
MutexLock mu(self, monitor_lock_);
while (true) {
if (owner_ == nullptr) { // Unowned.
owner_ = self;
CHECK_EQ(lock_count_, 0);
// When debugging, save the current monitor holder for future
// acquisition failures to use in sampled logging.
if (lock_profiling_threshold_ != 0) {
locking_method_ = self->GetCurrentMethod(&locking_dex_pc_);
}
return;
} else if (owner_ == self) { // Recursive.
lock_count_++;
return;
}
// Contended.
const bool log_contention = (lock_profiling_threshold_ != 0);
uint64_t wait_start_ms = log_contention ? MilliTime() : 0;
mirror::ArtMethod* owners_method = locking_method_;
uint32_t owners_dex_pc = locking_dex_pc_;
// Do this before releasing the lock so that we don't get deflated.
size_t num_waiters = num_waiters_;
++num_waiters_;
monitor_lock_.Unlock(self); // Let go of locks in order.
self->SetMonitorEnterObject(GetObject());
{
ScopedThreadStateChange tsc(self, kBlocked); // Change to blocked and give up mutator_lock_.
MutexLock mu2(self, monitor_lock_); // Reacquire monitor_lock_ without mutator_lock_ for Wait.
if (owner_ != NULL) { // Did the owner_ give the lock up?
monitor_contenders_.Wait(self); // Still contended so wait.
// Woken from contention.
if (log_contention) {
uint64_t wait_ms = MilliTime() - wait_start_ms;
uint32_t sample_percent;
if (wait_ms >= lock_profiling_threshold_) {
sample_percent = 100;
} else {
sample_percent = 100 * wait_ms / lock_profiling_threshold_;
}
if (sample_percent != 0 && (static_cast<uint32_t>(rand() % 100) < sample_percent)) {
const char* owners_filename;
uint32_t owners_line_number;
TranslateLocation(owners_method, owners_dex_pc, &owners_filename, &owners_line_number);
if (wait_ms > kLongWaitMs && owners_method != nullptr) {
LOG(WARNING) << "Long monitor contention event with owner method="
<< PrettyMethod(owners_method) << " from " << owners_filename << ":"
<< owners_line_number << " waiters=" << num_waiters << " for "
<< PrettyDuration(MsToNs(wait_ms));
}
LogContentionEvent(self, wait_ms, sample_percent, owners_filename, owners_line_number);
}
}
}
}
self->SetMonitorEnterObject(nullptr);
monitor_lock_.Lock(self); // Reacquire locks in order.
--num_waiters_;
}
}
static void ThrowIllegalMonitorStateExceptionF(const char* fmt, ...)
__attribute__((format(printf, 1, 2)));
static void ThrowIllegalMonitorStateExceptionF(const char* fmt, ...)
SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
va_list args;
va_start(args, fmt);
Thread* self = Thread::Current();
ThrowLocation throw_location = self->GetCurrentLocationForThrow();
self->ThrowNewExceptionV(throw_location, "Ljava/lang/IllegalMonitorStateException;", fmt, args);
if (!Runtime::Current()->IsStarted() || VLOG_IS_ON(monitor)) {
std::ostringstream ss;
self->Dump(ss);
LOG(Runtime::Current()->IsStarted() ? INFO : ERROR)
<< self->GetException(NULL)->Dump() << "\n" << ss.str();
}
va_end(args);
}
static std::string ThreadToString(Thread* thread) {
if (thread == NULL) {
return "NULL";
}
std::ostringstream oss;
// TODO: alternatively, we could just return the thread's name.
oss << *thread;
return oss.str();
}
void Monitor::FailedUnlock(mirror::Object* o, Thread* expected_owner, Thread* found_owner,
Monitor* monitor) {
Thread* current_owner = NULL;
std::string current_owner_string;
std::string expected_owner_string;
std::string found_owner_string;
{
// TODO: isn't this too late to prevent threads from disappearing?
// Acquire thread list lock so threads won't disappear from under us.
MutexLock mu(Thread::Current(), *Locks::thread_list_lock_);
// Re-read owner now that we hold lock.
current_owner = (monitor != NULL) ? monitor->GetOwner() : NULL;
// Get short descriptions of the threads involved.
current_owner_string = ThreadToString(current_owner);
expected_owner_string = ThreadToString(expected_owner);
found_owner_string = ThreadToString(found_owner);
}
if (current_owner == NULL) {
if (found_owner == NULL) {
ThrowIllegalMonitorStateExceptionF("unlock of unowned monitor on object of type '%s'"
" on thread '%s'",
PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
} else {
// Race: the original read found an owner but now there is none
ThrowIllegalMonitorStateExceptionF("unlock of monitor owned by '%s' on object of type '%s'"
" (where now the monitor appears unowned) on thread '%s'",
found_owner_string.c_str(),
PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
}
} else {
if (found_owner == NULL) {
// Race: originally there was no owner, there is now
ThrowIllegalMonitorStateExceptionF("unlock of monitor owned by '%s' on object of type '%s'"
" (originally believed to be unowned) on thread '%s'",
current_owner_string.c_str(),
PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
} else {
if (found_owner != current_owner) {
// Race: originally found and current owner have changed
ThrowIllegalMonitorStateExceptionF("unlock of monitor originally owned by '%s' (now"
" owned by '%s') on object of type '%s' on thread '%s'",
found_owner_string.c_str(),
current_owner_string.c_str(),
PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
} else {
ThrowIllegalMonitorStateExceptionF("unlock of monitor owned by '%s' on object of type '%s'"
" on thread '%s",
current_owner_string.c_str(),
PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
}
}
}
}
bool Monitor::Unlock(Thread* self) {
DCHECK(self != NULL);
MutexLock mu(self, monitor_lock_);
Thread* owner = owner_;
if (owner == self) {
// We own the monitor, so nobody else can be in here.
if (lock_count_ == 0) {
owner_ = NULL;
locking_method_ = NULL;
locking_dex_pc_ = 0;
// Wake a contender.
monitor_contenders_.Signal(self);
} else {
--lock_count_;
}
} else {
// We don't own this, so we're not allowed to unlock it.
// The JNI spec says that we should throw IllegalMonitorStateException
// in this case.
FailedUnlock(GetObject(), self, owner, this);
return false;
}
return true;
}
/*
* 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 Monitor::Wait(Thread* self, int64_t ms, int32_t ns,
bool interruptShouldThrow, ThreadState why) {
DCHECK(self != NULL);
DCHECK(why == kTimedWaiting || why == kWaiting || why == kSleeping);
monitor_lock_.Lock(self);
// Make sure that we hold the lock.
if (owner_ != self) {
monitor_lock_.Unlock(self);
ThrowIllegalMonitorStateExceptionF("object not locked by thread before wait()");
return;
}
// We need to turn a zero-length timed wait into a regular wait because
// Object.wait(0, 0) is defined as Object.wait(0), which is defined as Object.wait().
if (why == kTimedWaiting && (ms == 0 && ns == 0)) {
why = kWaiting;
}
// Enforce the timeout range.
if (ms < 0 || ns < 0 || ns > 999999) {
monitor_lock_.Unlock(self);
ThrowLocation throw_location = self->GetCurrentLocationForThrow();
self->ThrowNewExceptionF(throw_location, "Ljava/lang/IllegalArgumentException;",
"timeout arguments out of range: ms=%" PRId64 " ns=%d", ms, ns);
return;
}
/*
* Add ourselves to the set of threads waiting on this monitor, and
* release our hold. We need to let it go even if we're a few levels
* deep in a recursive lock, and we need to restore that later.
*
* We append to the wait set ahead of clearing the count and owner
* fields so the subroutine can check that the calling thread owns
* the monitor. Aside from that, the order of member updates is
* not order sensitive as we hold the pthread mutex.
*/
AppendToWaitSet(self);
++num_waiters_;
int prev_lock_count = lock_count_;
lock_count_ = 0;
owner_ = NULL;
mirror::ArtMethod* saved_method = locking_method_;
locking_method_ = NULL;
uintptr_t saved_dex_pc = locking_dex_pc_;
locking_dex_pc_ = 0;
/*
* Update thread state. If the GC wakes up, it'll ignore us, knowing
* that we won't touch any references in this state, and we'll check
* our suspend mode before we transition out.
*/
self->TransitionFromRunnableToSuspended(why);
bool was_interrupted = false;
{
// Pseudo-atomically wait on self's wait_cond_ and release the monitor lock.
MutexLock mu(self, *self->GetWaitMutex());
// Set wait_monitor_ to the monitor object we will be waiting on. When wait_monitor_ is
// non-NULL a notifying or interrupting thread must signal the thread's wait_cond_ to wake it
// up.
DCHECK(self->GetWaitMonitor() == nullptr);
self->SetWaitMonitor(this);
// Release the monitor lock.
monitor_contenders_.Signal(self);
monitor_lock_.Unlock(self);
// Handle the case where the thread was interrupted before we called wait().
if (self->IsInterruptedLocked()) {
was_interrupted = true;
} else {
// Wait for a notification or a timeout to occur.
if (why == kWaiting) {
self->GetWaitConditionVariable()->Wait(self);
} else {
DCHECK(why == kTimedWaiting || why == kSleeping) << why;
self->GetWaitConditionVariable()->TimedWait(self, ms, ns);
}
if (self->IsInterruptedLocked()) {
was_interrupted = true;
}
self->SetInterruptedLocked(false);
}
}
// Set self->status back to kRunnable, and self-suspend if needed.
self->TransitionFromSuspendedToRunnable();
{
// We reset the thread's wait_monitor_ field after transitioning back to runnable so
// that a thread in a waiting/sleeping state has a non-null wait_monitor_ for debugging
// and diagnostic purposes. (If you reset this earlier, stack dumps will claim that threads
// are waiting on "null".)
MutexLock mu(self, *self->GetWaitMutex());
DCHECK(self->GetWaitMonitor() != nullptr);
self->SetWaitMonitor(nullptr);
}
// Re-acquire the monitor and lock.
Lock(self);
monitor_lock_.Lock(self);
self->GetWaitMutex()->AssertNotHeld(self);
/*
* We remove our thread from wait set after restoring the count
* and owner fields so the subroutine can check that the calling
* thread owns the monitor. Aside from that, the order of member
* updates is not order sensitive as we hold the pthread mutex.
*/
owner_ = self;
lock_count_ = prev_lock_count;
locking_method_ = saved_method;
locking_dex_pc_ = saved_dex_pc;
--num_waiters_;
RemoveFromWaitSet(self);
monitor_lock_.Unlock(self);
if (was_interrupted) {
/*
* We were interrupted while waiting, or somebody interrupted an
* un-interruptible thread earlier and we're bailing out immediately.
*
* The doc sayeth: "The interrupted status of the current thread is
* cleared when this exception is thrown."
*/
{
MutexLock mu(self, *self->GetWaitMutex());
self->SetInterruptedLocked(false);
}
if (interruptShouldThrow) {
ThrowLocation throw_location = self->GetCurrentLocationForThrow();
self->ThrowNewException(throw_location, "Ljava/lang/InterruptedException;", NULL);
}
}
}
void Monitor::Notify(Thread* self) {
DCHECK(self != NULL);
MutexLock mu(self, monitor_lock_);
// Make sure that we hold the lock.
if (owner_ != self) {
ThrowIllegalMonitorStateExceptionF("object not locked by thread before notify()");
return;
}
// Signal the first waiting thread in the wait set.
while (wait_set_ != NULL) {
Thread* thread = wait_set_;
wait_set_ = thread->GetWaitNext();
thread->SetWaitNext(nullptr);
// Check to see if the thread is still waiting.
MutexLock mu(self, *thread->GetWaitMutex());
if (thread->GetWaitMonitor() != nullptr) {
thread->GetWaitConditionVariable()->Signal(self);
return;
}
}
}
void Monitor::NotifyAll(Thread* self) {
DCHECK(self != NULL);
MutexLock mu(self, monitor_lock_);
// Make sure that we hold the lock.
if (owner_ != self) {
ThrowIllegalMonitorStateExceptionF("object not locked by thread before notifyAll()");
return;
}
// Signal all threads in the wait set.
while (wait_set_ != NULL) {
Thread* thread = wait_set_;
wait_set_ = thread->GetWaitNext();
thread->SetWaitNext(nullptr);
thread->Notify();
}
}
bool Monitor::Deflate(Thread* self, mirror::Object* obj) {
DCHECK(obj != nullptr);
// Don't need volatile since we only deflate with mutators suspended.
LockWord lw(obj->GetLockWord(false));
// If the lock isn't an inflated monitor, then we don't need to deflate anything.
if (lw.GetState() == LockWord::kFatLocked) {
Monitor* monitor = lw.FatLockMonitor();
DCHECK(monitor != nullptr);
MutexLock mu(self, monitor->monitor_lock_);
// Can't deflate if we have anybody waiting on the CV.
if (monitor->num_waiters_ > 0) {
return false;
}
Thread* owner = monitor->owner_;
if (owner != nullptr) {
// Can't deflate if we are locked and have a hash code.
if (monitor->HasHashCode()) {
return false;
}
// Can't deflate if our lock count is too high.
if (monitor->lock_count_ > LockWord::kThinLockMaxCount) {
return false;
}
// Deflate to a thin lock.
obj->SetLockWord(LockWord::FromThinLockId(owner->GetThreadId(), monitor->lock_count_), false);
VLOG(monitor) << "Deflated " << obj << " to thin lock " << owner->GetTid() << " / "
<< monitor->lock_count_;
} else if (monitor->HasHashCode()) {
obj->SetLockWord(LockWord::FromHashCode(monitor->GetHashCode()), false);
VLOG(monitor) << "Deflated " << obj << " to hash monitor " << monitor->GetHashCode();
} else {
// No lock and no hash, just put an empty lock word inside the object.
obj->SetLockWord(LockWord(), false);
VLOG(monitor) << "Deflated" << obj << " to empty lock word";
}
// The monitor is deflated, mark the object as nullptr so that we know to delete it during the
// next GC.
monitor->obj_ = GcRoot<mirror::Object>(nullptr);
}
return true;
}
void Monitor::Inflate(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code) {
DCHECK(self != nullptr);
DCHECK(obj != nullptr);
// Allocate and acquire a new monitor.
Monitor* m = MonitorPool::CreateMonitor(self, owner, obj, hash_code);
DCHECK(m != nullptr);
if (m->Install(self)) {
if (owner != nullptr) {
VLOG(monitor) << "monitor: thread" << owner->GetThreadId()
<< " created monitor " << m << " for object " << obj;
} else {
VLOG(monitor) << "monitor: Inflate with hashcode " << hash_code
<< " created monitor " << m << " for object " << obj;
}
Runtime::Current()->GetMonitorList()->Add(m);
CHECK_EQ(obj->GetLockWord(true).GetState(), LockWord::kFatLocked);
} else {
MonitorPool::ReleaseMonitor(self, m);
}
}
void Monitor::InflateThinLocked(Thread* self, Handle<mirror::Object> obj, LockWord lock_word,
uint32_t hash_code) {
DCHECK_EQ(lock_word.GetState(), LockWord::kThinLocked);
uint32_t owner_thread_id = lock_word.ThinLockOwner();
if (owner_thread_id == self->GetThreadId()) {
// We own the monitor, we can easily inflate it.
Inflate(self, self, obj.Get(), hash_code);
} else {
ThreadList* thread_list = Runtime::Current()->GetThreadList();
// Suspend the owner, inflate. First change to blocked and give up mutator_lock_.
self->SetMonitorEnterObject(obj.Get());
bool timed_out;
Thread* owner;
{
ScopedThreadStateChange tsc(self, kBlocked);
// Take suspend thread lock to avoid races with threads trying to suspend this one.
MutexLock mu(self, *Locks::thread_list_suspend_thread_lock_);
owner = thread_list->SuspendThreadByThreadId(owner_thread_id, false, &timed_out);
}
if (owner != nullptr) {
// We succeeded in suspending the thread, check the lock's status didn't change.
lock_word = obj->GetLockWord(true);
if (lock_word.GetState() == LockWord::kThinLocked &&
lock_word.ThinLockOwner() == owner_thread_id) {
// Go ahead and inflate the lock.
Inflate(self, owner, obj.Get(), hash_code);
}
thread_list->Resume(owner, false);
}
self->SetMonitorEnterObject(nullptr);
}
}
// Fool annotalysis into thinking that the lock on obj is acquired.
static mirror::Object* FakeLock(mirror::Object* obj)
EXCLUSIVE_LOCK_FUNCTION(obj) NO_THREAD_SAFETY_ANALYSIS {
return obj;
}
// Fool annotalysis into thinking that the lock on obj is release.
static mirror::Object* FakeUnlock(mirror::Object* obj)
UNLOCK_FUNCTION(obj) NO_THREAD_SAFETY_ANALYSIS {
return obj;
}
mirror::Object* Monitor::MonitorEnter(Thread* self, mirror::Object* obj) {
DCHECK(self != NULL);
DCHECK(obj != NULL);
obj = FakeLock(obj);
uint32_t thread_id = self->GetThreadId();
size_t contention_count = 0;
StackHandleScope<1> hs(self);
Handle<mirror::Object> h_obj(hs.NewHandle(obj));
while (true) {
LockWord lock_word = h_obj->GetLockWord(true);
switch (lock_word.GetState()) {
case LockWord::kUnlocked: {
LockWord thin_locked(LockWord::FromThinLockId(thread_id, 0));
if (h_obj->CasLockWordWeakSequentiallyConsistent(lock_word, thin_locked)) {
// CasLockWord enforces more than the acquire ordering we need here.
return h_obj.Get(); // Success!
}
continue; // Go again.
}
case LockWord::kThinLocked: {
uint32_t owner_thread_id = lock_word.ThinLockOwner();
if (owner_thread_id == thread_id) {
// We own the lock, increase the recursion count.
uint32_t new_count = lock_word.ThinLockCount() + 1;
if (LIKELY(new_count <= LockWord::kThinLockMaxCount)) {
LockWord thin_locked(LockWord::FromThinLockId(thread_id, new_count));
h_obj->SetLockWord(thin_locked, true);
return h_obj.Get(); // Success!
} else {
// We'd overflow the recursion count, so inflate the monitor.
InflateThinLocked(self, h_obj, lock_word, 0);
}
} else {
// Contention.
contention_count++;
Runtime* runtime = Runtime::Current();
if (contention_count <= runtime->GetMaxSpinsBeforeThinkLockInflation()) {
// TODO: Consider switching the thread state to kBlocked when we are yielding.
// Use sched_yield instead of NanoSleep since NanoSleep can wait much longer than the
// parameter you pass in. This can cause thread suspension to take excessively long
// and make long pauses. See b/16307460.
sched_yield();
} else {
contention_count = 0;
InflateThinLocked(self, h_obj, lock_word, 0);
}
}
continue; // Start from the beginning.
}
case LockWord::kFatLocked: {
Monitor* mon = lock_word.FatLockMonitor();
mon->Lock(self);
return h_obj.Get(); // Success!
}
case LockWord::kHashCode:
// Inflate with the existing hashcode.
Inflate(self, nullptr, h_obj.Get(), lock_word.GetHashCode());
continue; // Start from the beginning.
default: {
LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
return h_obj.Get();
}
}
}
}
bool Monitor::MonitorExit(Thread* self, mirror::Object* obj) {
DCHECK(self != NULL);
DCHECK(obj != NULL);
obj = FakeUnlock(obj);
LockWord lock_word = obj->GetLockWord(true);
StackHandleScope<1> hs(self);
Handle<mirror::Object> h_obj(hs.NewHandle(obj));
switch (lock_word.GetState()) {
case LockWord::kHashCode:
// Fall-through.
case LockWord::kUnlocked:
FailedUnlock(h_obj.Get(), self, nullptr, nullptr);
return false; // Failure.
case LockWord::kThinLocked: {
uint32_t thread_id = self->GetThreadId();
uint32_t owner_thread_id = lock_word.ThinLockOwner();
if (owner_thread_id != thread_id) {
// TODO: there's a race here with the owner dying while we unlock.
Thread* owner =
Runtime::Current()->GetThreadList()->FindThreadByThreadId(lock_word.ThinLockOwner());
FailedUnlock(h_obj.Get(), self, owner, nullptr);
return false; // Failure.
} else {
// We own the lock, decrease the recursion count.
if (lock_word.ThinLockCount() != 0) {
uint32_t new_count = lock_word.ThinLockCount() - 1;
LockWord thin_locked(LockWord::FromThinLockId(thread_id, new_count));
h_obj->SetLockWord(thin_locked, true);
} else {
h_obj->SetLockWord(LockWord(), true);
}
return true; // Success!
}
}
case LockWord::kFatLocked: {
Monitor* mon = lock_word.FatLockMonitor();
return mon->Unlock(self);
}
default: {
LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
return false;
}
}
}
/*
* Object.wait(). Also called for class init.
*/
void Monitor::Wait(Thread* self, mirror::Object *obj, int64_t ms, int32_t ns,
bool interruptShouldThrow, ThreadState why) {
DCHECK(self != nullptr);
DCHECK(obj != nullptr);
LockWord lock_word = obj->GetLockWord(true);
while (lock_word.GetState() != LockWord::kFatLocked) {
switch (lock_word.GetState()) {
case LockWord::kHashCode:
// Fall-through.
case LockWord::kUnlocked:
ThrowIllegalMonitorStateExceptionF("object not locked by thread before wait()");
return; // Failure.
case LockWord::kThinLocked: {
uint32_t thread_id = self->GetThreadId();
uint32_t owner_thread_id = lock_word.ThinLockOwner();
if (owner_thread_id != thread_id) {
ThrowIllegalMonitorStateExceptionF("object not locked by thread before wait()");
return; // Failure.
} else {
// We own the lock, inflate to enqueue ourself on the Monitor. May fail spuriously so
// re-load.
Inflate(self, self, obj, 0);
lock_word = obj->GetLockWord(true);
}
break;
}
case LockWord::kFatLocked: // Unreachable given the loop condition above. Fall-through.
default: {
LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
return;
}
}
}
Monitor* mon = lock_word.FatLockMonitor();
mon->Wait(self, ms, ns, interruptShouldThrow, why);
}
void Monitor::DoNotify(Thread* self, mirror::Object* obj, bool notify_all) {
DCHECK(self != nullptr);
DCHECK(obj != nullptr);
LockWord lock_word = obj->GetLockWord(true);
switch (lock_word.GetState()) {
case LockWord::kHashCode:
// Fall-through.
case LockWord::kUnlocked:
ThrowIllegalMonitorStateExceptionF("object not locked by thread before notify()");
return; // Failure.
case LockWord::kThinLocked: {
uint32_t thread_id = self->GetThreadId();
uint32_t owner_thread_id = lock_word.ThinLockOwner();
if (owner_thread_id != thread_id) {
ThrowIllegalMonitorStateExceptionF("object not locked by thread before notify()");
return; // Failure.
} else {
// We own the lock but there's no Monitor and therefore no waiters.
return; // Success.
}
}
case LockWord::kFatLocked: {
Monitor* mon = lock_word.FatLockMonitor();
if (notify_all) {
mon->NotifyAll(self);
} else {
mon->Notify(self);
}
return; // Success.
}
default: {
LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
return;
}
}
}
uint32_t Monitor::GetLockOwnerThreadId(mirror::Object* obj) {
DCHECK(obj != nullptr);
LockWord lock_word = obj->GetLockWord(true);
switch (lock_word.GetState()) {
case LockWord::kHashCode:
// Fall-through.
case LockWord::kUnlocked:
return ThreadList::kInvalidThreadId;
case LockWord::kThinLocked:
return lock_word.ThinLockOwner();
case LockWord::kFatLocked: {
Monitor* mon = lock_word.FatLockMonitor();
return mon->GetOwnerThreadId();
}
default: {
LOG(FATAL) << "Unreachable";
return ThreadList::kInvalidThreadId;
}
}
}
void Monitor::DescribeWait(std::ostream& os, const Thread* thread) {
// Determine the wait message and object we're waiting or blocked upon.
mirror::Object* pretty_object = nullptr;
const char* wait_message = nullptr;
uint32_t lock_owner = ThreadList::kInvalidThreadId;
ThreadState state = thread->GetState();
if (state == kWaiting || state == kTimedWaiting || state == kSleeping) {
wait_message = (state == kSleeping) ? " - sleeping on " : " - waiting on ";
Thread* self = Thread::Current();
MutexLock mu(self, *thread->GetWaitMutex());
Monitor* monitor = thread->GetWaitMonitor();
if (monitor != nullptr) {
pretty_object = monitor->GetObject();
}
} else if (state == kBlocked) {
wait_message = " - waiting to lock ";
pretty_object = thread->GetMonitorEnterObject();
if (pretty_object != nullptr) {
lock_owner = pretty_object->GetLockOwnerThreadId();
}
}
if (wait_message != nullptr) {
if (pretty_object == nullptr) {
os << wait_message << "an unknown object";
} else {
if ((pretty_object->GetLockWord(true).GetState() == LockWord::kThinLocked) &&
Locks::mutator_lock_->IsExclusiveHeld(Thread::Current())) {
// Getting the identity hashcode here would result in lock inflation and suspension of the
// current thread, which isn't safe if this is the only runnable thread.
os << wait_message << StringPrintf("<@addr=0x%" PRIxPTR "> (a %s)",
reinterpret_cast<intptr_t>(pretty_object),
PrettyTypeOf(pretty_object).c_str());
} else {
// - waiting on <0x6008c468> (a java.lang.Class<java.lang.ref.ReferenceQueue>)
os << wait_message << StringPrintf("<0x%08x> (a %s)", pretty_object->IdentityHashCode(),
PrettyTypeOf(pretty_object).c_str());
}
}
// - waiting to lock <0x613f83d8> (a java.lang.Object) held by thread 5
if (lock_owner != ThreadList::kInvalidThreadId) {
os << " held by thread " << lock_owner;
}
os << "\n";
}
}
mirror::Object* Monitor::GetContendedMonitor(Thread* thread) {
// This is used to implement JDWP's ThreadReference.CurrentContendedMonitor, and has a bizarre
// definition of contended that includes a monitor a thread is trying to enter...
mirror::Object* result = thread->GetMonitorEnterObject();
if (result == NULL) {
// ...but also a monitor that the thread is waiting on.
MutexLock mu(Thread::Current(), *thread->GetWaitMutex());
Monitor* monitor = thread->GetWaitMonitor();
if (monitor != NULL) {
result = monitor->GetObject();
}
}
return result;
}
void Monitor::VisitLocks(StackVisitor* stack_visitor, void (*callback)(mirror::Object*, void*),
void* callback_context, bool abort_on_failure) {
mirror::ArtMethod* m = stack_visitor->GetMethod();
CHECK(m != NULL);
// Native methods are an easy special case.
// TODO: use the JNI implementation's table of explicit MonitorEnter calls and dump those too.
if (m->IsNative()) {
if (m->IsSynchronized()) {
mirror::Object* jni_this = stack_visitor->GetCurrentHandleScope()->GetReference(0);
callback(jni_this, callback_context);
}
return;
}
// Proxy methods should not be synchronized.
if (m->IsProxyMethod()) {
CHECK(!m->IsSynchronized());
return;
}
// Is there any reason to believe there's any synchronization in this method?
const DexFile::CodeItem* code_item = m->GetCodeItem();
CHECK(code_item != NULL) << PrettyMethod(m);
if (code_item->tries_size_ == 0) {
return; // No "tries" implies no synchronization, so no held locks to report.
}
// Get the dex pc. If abort_on_failure is false, GetDexPc will not abort in the case it cannot
// find the dex pc, and instead return kDexNoIndex. Then bail out, as it indicates we have an
// inconsistent stack anyways.
uint32_t dex_pc = stack_visitor->GetDexPc(abort_on_failure);
if (!abort_on_failure && dex_pc == DexFile::kDexNoIndex) {
LOG(ERROR) << "Could not find dex_pc for " << PrettyMethod(m);
return;
}
// Ask the verifier for the dex pcs of all the monitor-enter instructions corresponding to
// the locks held in this stack frame.
std::vector<uint32_t> monitor_enter_dex_pcs;
verifier::MethodVerifier::FindLocksAtDexPc(m, dex_pc, &monitor_enter_dex_pcs);
if (monitor_enter_dex_pcs.empty()) {
return;
}
for (size_t i = 0; i < monitor_enter_dex_pcs.size(); ++i) {
// The verifier works in terms of the dex pcs of the monitor-enter instructions.
// We want the registers used by those instructions (so we can read the values out of them).
uint32_t dex_pc = monitor_enter_dex_pcs[i];
uint16_t monitor_enter_instruction = code_item->insns_[dex_pc];
// Quick sanity check.
if ((monitor_enter_instruction & 0xff) != Instruction::MONITOR_ENTER) {
LOG(FATAL) << "expected monitor-enter @" << dex_pc << "; was "
<< reinterpret_cast<void*>(monitor_enter_instruction);
}
uint16_t monitor_register = ((monitor_enter_instruction >> 8) & 0xff);
mirror::Object* o = reinterpret_cast<mirror::Object*>(stack_visitor->GetVReg(m, monitor_register,
kReferenceVReg));
callback(o, callback_context);
}
}
bool Monitor::IsValidLockWord(LockWord lock_word) {
switch (lock_word.GetState()) {
case LockWord::kUnlocked:
// Nothing to check.
return true;
case LockWord::kThinLocked:
// Basic sanity check of owner.
return lock_word.ThinLockOwner() != ThreadList::kInvalidThreadId;
case LockWord::kFatLocked: {
// Check the monitor appears in the monitor list.
Monitor* mon = lock_word.FatLockMonitor();
MonitorList* list = Runtime::Current()->GetMonitorList();
MutexLock mu(Thread::Current(), list->monitor_list_lock_);
for (Monitor* list_mon : list->list_) {
if (mon == list_mon) {
return true; // Found our monitor.
}
}
return false; // Fail - unowned monitor in an object.
}
case LockWord::kHashCode:
return true;
default:
LOG(FATAL) << "Unreachable";
return false;
}
}
bool Monitor::IsLocked() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
MutexLock mu(Thread::Current(), monitor_lock_);
return owner_ != nullptr;
}
void Monitor::TranslateLocation(mirror::ArtMethod* method, uint32_t dex_pc,
const char** source_file, uint32_t* line_number) const {
// If method is null, location is unknown
if (method == NULL) {
*source_file = "";
*line_number = 0;
return;
}
*source_file = method->GetDeclaringClassSourceFile();
if (*source_file == NULL) {
*source_file = "";
}
*line_number = method->GetLineNumFromDexPC(dex_pc);
}
uint32_t Monitor::GetOwnerThreadId() {
MutexLock mu(Thread::Current(), monitor_lock_);
Thread* owner = owner_;
if (owner != NULL) {
return owner->GetThreadId();
} else {
return ThreadList::kInvalidThreadId;
}
}
MonitorList::MonitorList()
: allow_new_monitors_(true), monitor_list_lock_("MonitorList lock", kMonitorListLock),
monitor_add_condition_("MonitorList disallow condition", monitor_list_lock_) {
}
MonitorList::~MonitorList() {
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
// Release all monitors to the pool.
// TODO: Is it an invariant that *all* open monitors are in the list? Then we could
// clear faster in the pool.
MonitorPool::ReleaseMonitors(self, &list_);
}
void MonitorList::DisallowNewMonitors() {
MutexLock mu(Thread::Current(), monitor_list_lock_);
allow_new_monitors_ = false;
}
void MonitorList::AllowNewMonitors() {
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
allow_new_monitors_ = true;
monitor_add_condition_.Broadcast(self);
}
void MonitorList::Add(Monitor* m) {
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
while (UNLIKELY(!allow_new_monitors_)) {
monitor_add_condition_.WaitHoldingLocks(self);
}
list_.push_front(m);
}
void MonitorList::SweepMonitorList(IsMarkedCallback* callback, void* arg) {
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
for (auto it = list_.begin(); it != list_.end(); ) {
Monitor* m = *it;
// Disable the read barrier in GetObject() as this is called by GC.
mirror::Object* obj = m->GetObject<kWithoutReadBarrier>();
// The object of a monitor can be null if we have deflated it.
mirror::Object* new_obj = obj != nullptr ? callback(obj, arg) : nullptr;
if (new_obj == nullptr) {
VLOG(monitor) << "freeing monitor " << m << " belonging to unmarked object "
<< obj;
MonitorPool::ReleaseMonitor(self, m);
it = list_.erase(it);
} else {
m->SetObject(new_obj);
++it;
}
}
}
struct MonitorDeflateArgs {
MonitorDeflateArgs() : self(Thread::Current()), deflate_count(0) {}
Thread* const self;
size_t deflate_count;
};
static mirror::Object* MonitorDeflateCallback(mirror::Object* object, void* arg)
SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
MonitorDeflateArgs* args = reinterpret_cast<MonitorDeflateArgs*>(arg);
if (Monitor::Deflate(args->self, object)) {
DCHECK_NE(object->GetLockWord(true).GetState(), LockWord::kFatLocked);
++args->deflate_count;
// If we deflated, return nullptr so that the monitor gets removed from the array.
return nullptr;
}
return object; // Monitor was not deflated.
}
size_t MonitorList::DeflateMonitors() {
MonitorDeflateArgs args;
Locks::mutator_lock_->AssertExclusiveHeld(args.self);
SweepMonitorList(MonitorDeflateCallback, &args);
return args.deflate_count;
}
MonitorInfo::MonitorInfo(mirror::Object* obj) : owner_(NULL), entry_count_(0) {
DCHECK(obj != nullptr);
LockWord lock_word = obj->GetLockWord(true);
switch (lock_word.GetState()) {
case LockWord::kUnlocked:
// Fall-through.
case LockWord::kForwardingAddress:
// Fall-through.
case LockWord::kHashCode:
break;
case LockWord::kThinLocked:
owner_ = Runtime::Current()->GetThreadList()->FindThreadByThreadId(lock_word.ThinLockOwner());
entry_count_ = 1 + lock_word.ThinLockCount();
// Thin locks have no waiters.
break;
case LockWord::kFatLocked: {
Monitor* mon = lock_word.FatLockMonitor();
owner_ = mon->owner_;
entry_count_ = 1 + mon->lock_count_;
for (Thread* waiter = mon->wait_set_; waiter != NULL; waiter = waiter->GetWaitNext()) {
waiters_.push_back(waiter);
}
break;
}
}
}
} // namespace art