/*
* 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-inl.h"
#include <vector>
#include "android-base/stringprintf.h"
#include "art_method-inl.h"
#include "base/logging.h" // For VLOG.
#include "base/mutex.h"
#include "base/quasi_atomic.h"
#include "base/stl_util.h"
#include "base/systrace.h"
#include "base/time_utils.h"
#include "class_linker.h"
#include "dex/dex_file-inl.h"
#include "dex/dex_file_types.h"
#include "dex/dex_instruction-inl.h"
#include "lock_word-inl.h"
#include "mirror/class-inl.h"
#include "mirror/object-inl.h"
#include "object_callbacks.h"
#include "scoped_thread_state_change-inl.h"
#include "stack.h"
#include "thread.h"
#include "thread_list.h"
#include "verifier/method_verifier.h"
#include "well_known_classes.h"
namespace art {
using android::base::StringPrintf;
static constexpr uint64_t kDebugThresholdFudgeFactor = kIsDebugBuild ? 10 : 1;
static constexpr uint64_t kLongWaitMs = 100 * kDebugThresholdFudgeFactor;
/*
* 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. We
* deflate locks from time to time as part of heap trimming.
*
* 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.
*/
uint32_t Monitor::lock_profiling_threshold_ = 0;
uint32_t Monitor::stack_dump_lock_profiling_threshold_ = 0;
void Monitor::Init(uint32_t lock_profiling_threshold,
uint32_t stack_dump_lock_profiling_threshold) {
// It isn't great to always include the debug build fudge factor for command-
// line driven arguments, but it's easier to adjust here than in the build.
lock_profiling_threshold_ =
lock_profiling_threshold * kDebugThresholdFudgeFactor;
stack_dump_lock_profiling_threshold_ =
stack_dump_lock_profiling_threshold * kDebugThresholdFudgeFactor;
}
Monitor::Monitor(Thread* self, Thread* owner, ObjPtr<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_(nullptr),
wake_set_(nullptr),
hash_code_(hash_code),
locking_method_(nullptr),
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,
ObjPtr<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_(nullptr),
wake_set_(nullptr),
hash_code_(hash_code),
locking_method_(nullptr),
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() {
int32_t hc = hash_code_.load(std::memory_order_relaxed);
if (!HasHashCode()) {
// Use a strong CAS to prevent spurious failures since these can make the boot image
// non-deterministic.
hash_code_.CompareAndSetStrongRelaxed(0, mirror::Object::GenerateIdentityHashCode());
hc = hash_code_.load(std::memory_order_relaxed);
}
DCHECK(HasHashCode());
return hc;
}
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_.load(std::memory_order_relaxed), 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";
UNREACHABLE();
}
default: {
LOG(FATAL) << "Invalid monitor state " << lw.GetState();
UNREACHABLE();
}
}
LockWord fat(this, lw.GCState());
// Publish the updated lock word, which may race with other threads.
bool success = GetObject()->CasLockWord(lw, fat, CASMode::kWeak, std::memory_order_release);
// 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);
if (locking_method_ != nullptr && UNLIKELY(locking_method_->IsProxyMethod())) {
// Grab another frame. Proxy methods are not helpful for lock profiling. This should be rare
// enough that it's OK to walk the stack twice.
struct NextMethodVisitor final : public StackVisitor {
explicit NextMethodVisitor(Thread* thread) REQUIRES_SHARED(Locks::mutator_lock_)
: StackVisitor(thread,
nullptr,
StackVisitor::StackWalkKind::kIncludeInlinedFrames,
false),
count_(0),
method_(nullptr),
dex_pc_(0) {}
bool VisitFrame() override REQUIRES_SHARED(Locks::mutator_lock_) {
ArtMethod* m = GetMethod();
if (m->IsRuntimeMethod()) {
// Continue if this is a runtime method.
return true;
}
count_++;
if (count_ == 2u) {
method_ = m;
dex_pc_ = GetDexPc(false);
return false;
}
return true;
}
size_t count_;
ArtMethod* method_;
uint32_t dex_pc_;
};
NextMethodVisitor nmv(owner_);
nmv.WalkStack();
locking_method_ = nmv.method_;
locking_dex_pc_ = nmv.dex_pc_;
}
DCHECK(locking_method_ == nullptr || !locking_method_->IsProxyMethod());
}
return success;
}
Monitor::~Monitor() {
// Deflated monitors have a null object.
}
void Monitor::AppendToWaitSet(Thread* thread) {
// Not checking that the owner is equal to this thread, since we've released
// the monitor by the time this method is called.
DCHECK(thread != nullptr);
DCHECK(thread->GetWaitNext() == nullptr) << thread->GetWaitNext();
if (wait_set_ == nullptr) {
wait_set_ = thread;
return;
}
// push_back.
Thread* t = wait_set_;
while (t->GetWaitNext() != nullptr) {
t = t->GetWaitNext();
}
t->SetWaitNext(thread);
}
void Monitor::RemoveFromWaitSet(Thread *thread) {
DCHECK(owner_ == Thread::Current());
DCHECK(thread != nullptr);
auto remove = [&](Thread*& set){
if (set != nullptr) {
if (set == thread) {
set = thread->GetWaitNext();
thread->SetWaitNext(nullptr);
return true;
}
Thread* t = set;
while (t->GetWaitNext() != nullptr) {
if (t->GetWaitNext() == thread) {
t->SetWaitNext(thread->GetWaitNext());
thread->SetWaitNext(nullptr);
return true;
}
t = t->GetWaitNext();
}
}
return false;
};
if (remove(wait_set_)) {
return;
}
remove(wake_set_);
}
void Monitor::SetObject(ObjPtr<mirror::Object> object) {
obj_ = GcRoot<mirror::Object>(object);
}
// This function is inlined and just helps to not have the VLOG and ATRACE check at all the
// potential tracing points.
void Monitor::AtraceMonitorLock(Thread* self, ObjPtr<mirror::Object> obj, bool is_wait) {
if (UNLIKELY(VLOG_IS_ON(systrace_lock_logging) && ATraceEnabled())) {
AtraceMonitorLockImpl(self, obj, is_wait);
}
}
void Monitor::AtraceMonitorLockImpl(Thread* self, ObjPtr<mirror::Object> obj, bool is_wait) {
// Wait() requires a deeper call stack to be useful. Otherwise you'll see "Waiting at
// Object.java". Assume that we'll wait a nontrivial amount, so it's OK to do a longer
// stack walk than if !is_wait.
const size_t wanted_frame_number = is_wait ? 1U : 0U;
ArtMethod* method = nullptr;
uint32_t dex_pc = 0u;
size_t current_frame_number = 0u;
StackVisitor::WalkStack(
// Note: Adapted from CurrentMethodVisitor in thread.cc. We must not resolve here.
[&](const art::StackVisitor* stack_visitor) REQUIRES_SHARED(Locks::mutator_lock_) {
ArtMethod* m = stack_visitor->GetMethod();
if (m == nullptr || m->IsRuntimeMethod()) {
// Runtime method, upcall, or resolution issue. Skip.
return true;
}
// Is this the requested frame?
if (current_frame_number == wanted_frame_number) {
method = m;
dex_pc = stack_visitor->GetDexPc(false /* abort_on_error*/);
return false;
}
// Look for more.
current_frame_number++;
return true;
},
self,
/* context= */ nullptr,
art::StackVisitor::StackWalkKind::kIncludeInlinedFrames);
const char* prefix = is_wait ? "Waiting on " : "Locking ";
const char* filename;
int32_t line_number;
TranslateLocation(method, dex_pc, &filename, &line_number);
// It would be nice to have a stable "ID" for the object here. However, the only stable thing
// would be the identity hashcode. But we cannot use IdentityHashcode here: For one, there are
// times when it is unsafe to make that call (see stack dumping for an explanation). More
// importantly, we would have to give up on thin-locking when adding systrace locks, as the
// identity hashcode is stored in the lockword normally (so can't be used with thin-locks).
//
// Because of thin-locks we also cannot use the monitor id (as there is no monitor). Monitor ids
// also do not have to be stable, as the monitor may be deflated.
std::string tmp = StringPrintf("%s %d at %s:%d",
prefix,
(obj == nullptr ? -1 : static_cast<int32_t>(reinterpret_cast<uintptr_t>(obj.Ptr()))),
(filename != nullptr ? filename : "null"),
line_number);
ATraceBegin(tmp.c_str());
}
void Monitor::AtraceMonitorUnlock() {
if (UNLIKELY(VLOG_IS_ON(systrace_lock_logging))) {
ATraceEnd();
}
}
std::string Monitor::PrettyContentionInfo(const std::string& owner_name,
pid_t owner_tid,
ArtMethod* owners_method,
uint32_t owners_dex_pc,
size_t num_waiters) {
Locks::mutator_lock_->AssertSharedHeld(Thread::Current());
const char* owners_filename;
int32_t owners_line_number = 0;
if (owners_method != nullptr) {
TranslateLocation(owners_method, owners_dex_pc, &owners_filename, &owners_line_number);
}
std::ostringstream oss;
oss << "monitor contention with owner " << owner_name << " (" << owner_tid << ")";
if (owners_method != nullptr) {
oss << " at " << owners_method->PrettyMethod();
oss << "(" << owners_filename << ":" << owners_line_number << ")";
}
oss << " waiters=" << num_waiters;
return oss.str();
}
bool Monitor::TryLockLocked(Thread* self) {
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_);
// We don't expect a proxy method here.
DCHECK(locking_method_ == nullptr || !locking_method_->IsProxyMethod());
}
} else if (owner_ == self) { // Recursive.
lock_count_++;
} else {
return false;
}
AtraceMonitorLock(self, GetObject(), /* is_wait= */ false);
return true;
}
bool Monitor::TryLock(Thread* self) {
MutexLock mu(self, monitor_lock_);
return TryLockLocked(self);
}
// Asserts that a mutex isn't held when the class comes into and out of scope.
class ScopedAssertNotHeld {
public:
ScopedAssertNotHeld(Thread* self, Mutex& mu) : self_(self), mu_(mu) {
mu_.AssertNotHeld(self_);
}
~ScopedAssertNotHeld() {
mu_.AssertNotHeld(self_);
}
private:
Thread* const self_;
Mutex& mu_;
DISALLOW_COPY_AND_ASSIGN(ScopedAssertNotHeld);
};
template <LockReason reason>
void Monitor::Lock(Thread* self) {
ScopedAssertNotHeld sanh(self, monitor_lock_);
bool called_monitors_callback = false;
monitor_lock_.Lock(self);
while (true) {
if (TryLockLocked(self)) {
break;
}
// Contended.
const bool log_contention = (lock_profiling_threshold_ != 0);
uint64_t wait_start_ms = log_contention ? MilliTime() : 0;
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_;
// If systrace logging is enabled, first look at the lock owner. Acquiring the monitor's
// lock and then re-acquiring the mutator lock can deadlock.
bool started_trace = false;
if (ATraceEnabled()) {
if (owner_ != nullptr) { // Did the owner_ give the lock up?
std::ostringstream oss;
std::string name;
owner_->GetThreadName(name);
oss << PrettyContentionInfo(name,
owner_->GetTid(),
owners_method,
owners_dex_pc,
num_waiters);
// Add info for contending thread.
uint32_t pc;
ArtMethod* m = self->GetCurrentMethod(&pc);
const char* filename;
int32_t line_number;
TranslateLocation(m, pc, &filename, &line_number);
oss << " blocking from "
<< ArtMethod::PrettyMethod(m) << "(" << (filename != nullptr ? filename : "null")
<< ":" << line_number << ")";
ATraceBegin(oss.str().c_str());
started_trace = true;
}
}
monitor_lock_.Unlock(self); // Let go of locks in order.
// Call the contended locking cb once and only once. Also only call it if we are locking for
// the first time, not during a Wait wakeup.
if (reason == LockReason::kForLock && !called_monitors_callback) {
called_monitors_callback = true;
Runtime::Current()->GetRuntimeCallbacks()->MonitorContendedLocking(this);
}
self->SetMonitorEnterObject(GetObject().Ptr());
{
ScopedThreadSuspension tsc(self, kBlocked); // Change to blocked and give up mutator_lock_.
uint32_t original_owner_thread_id = 0u;
{
// Reacquire monitor_lock_ without mutator_lock_ for Wait.
MutexLock mu2(self, monitor_lock_);
if (owner_ != nullptr) { // Did the owner_ give the lock up?
original_owner_thread_id = owner_->GetThreadId();
monitor_contenders_.Wait(self); // Still contended so wait.
}
}
if (original_owner_thread_id != 0u) {
// 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)) {
// Reacquire mutator_lock_ for logging.
ScopedObjectAccess soa(self);
bool owner_alive = false;
pid_t original_owner_tid = 0;
std::string original_owner_name;
const bool should_dump_stacks = stack_dump_lock_profiling_threshold_ > 0 &&
wait_ms > stack_dump_lock_profiling_threshold_;
std::string owner_stack_dump;
// Acquire thread-list lock to find thread and keep it from dying until we've got all
// the info we need.
{
Locks::thread_list_lock_->ExclusiveLock(Thread::Current());
// Re-find the owner in case the thread got killed.
Thread* original_owner = Runtime::Current()->GetThreadList()->FindThreadByThreadId(
original_owner_thread_id);
if (original_owner != nullptr) {
owner_alive = true;
original_owner_tid = original_owner->GetTid();
original_owner->GetThreadName(original_owner_name);
if (should_dump_stacks) {
// Very long contention. Dump stacks.
struct CollectStackTrace : public Closure {
void Run(art::Thread* thread) override
REQUIRES_SHARED(art::Locks::mutator_lock_) {
thread->DumpJavaStack(oss);
}
std::ostringstream oss;
};
CollectStackTrace owner_trace;
// RequestSynchronousCheckpoint releases the thread_list_lock_ as a part of its
// execution.
original_owner->RequestSynchronousCheckpoint(&owner_trace);
owner_stack_dump = owner_trace.oss.str();
} else {
Locks::thread_list_lock_->ExclusiveUnlock(Thread::Current());
}
} else {
Locks::thread_list_lock_->ExclusiveUnlock(Thread::Current());
}
// This is all the data we need. Now drop the thread-list lock, it's OK for the
// owner to go away now.
}
// If we found the owner (and thus have owner data), go and log now.
if (owner_alive) {
// Give the detailed traces for really long contention.
if (should_dump_stacks) {
// This must be here (and not above) because we cannot hold the thread-list lock
// while running the checkpoint.
std::ostringstream self_trace_oss;
self->DumpJavaStack(self_trace_oss);
uint32_t pc;
ArtMethod* m = self->GetCurrentMethod(&pc);
LOG(WARNING) << "Long "
<< PrettyContentionInfo(original_owner_name,
original_owner_tid,
owners_method,
owners_dex_pc,
num_waiters)
<< " in " << ArtMethod::PrettyMethod(m) << " for "
<< PrettyDuration(MsToNs(wait_ms)) << "\n"
<< "Current owner stack:\n" << owner_stack_dump
<< "Contender stack:\n" << self_trace_oss.str();
} else if (wait_ms > kLongWaitMs && owners_method != nullptr) {
uint32_t pc;
ArtMethod* m = self->GetCurrentMethod(&pc);
// TODO: We should maybe check that original_owner is still a live thread.
LOG(WARNING) << "Long "
<< PrettyContentionInfo(original_owner_name,
original_owner_tid,
owners_method,
owners_dex_pc,
num_waiters)
<< " in " << ArtMethod::PrettyMethod(m) << " for "
<< PrettyDuration(MsToNs(wait_ms));
}
LogContentionEvent(self,
wait_ms,
sample_percent,
owners_method,
owners_dex_pc);
}
}
}
}
}
if (started_trace) {
ATraceEnd();
}
self->SetMonitorEnterObject(nullptr);
monitor_lock_.Lock(self); // Reacquire locks in order.
--num_waiters_;
}
monitor_lock_.Unlock(self);
// We need to pair this with a single contended locking call. NB we match the RI behavior and call
// this even if MonitorEnter failed.
if (called_monitors_callback) {
CHECK(reason == LockReason::kForLock);
Runtime::Current()->GetRuntimeCallbacks()->MonitorContendedLocked(this);
}
}
template void Monitor::Lock<LockReason::kForLock>(Thread* self);
template void Monitor::Lock<LockReason::kForWait>(Thread* self);
static void ThrowIllegalMonitorStateExceptionF(const char* fmt, ...)
__attribute__((format(printf, 1, 2)));
static void ThrowIllegalMonitorStateExceptionF(const char* fmt, ...)
REQUIRES_SHARED(Locks::mutator_lock_) {
va_list args;
va_start(args, fmt);
Thread* self = Thread::Current();
self->ThrowNewExceptionV("Ljava/lang/IllegalMonitorStateException;", fmt, args);
if (!Runtime::Current()->IsStarted() || VLOG_IS_ON(monitor)) {
std::ostringstream ss;
self->Dump(ss);
LOG(Runtime::Current()->IsStarted() ? ::android::base::INFO : ::android::base::ERROR)
<< self->GetException()->Dump() << "\n" << ss.str();
}
va_end(args);
}
static std::string ThreadToString(Thread* thread) {
if (thread == nullptr) {
return "nullptr";
}
std::ostringstream oss;
// TODO: alternatively, we could just return the thread's name.
oss << *thread;
return oss.str();
}
void Monitor::FailedUnlock(ObjPtr<mirror::Object> o,
uint32_t expected_owner_thread_id,
uint32_t found_owner_thread_id,
Monitor* monitor) {
// Acquire thread list lock so threads won't disappear from under us.
std::string current_owner_string;
std::string expected_owner_string;
std::string found_owner_string;
uint32_t current_owner_thread_id = 0u;
{
MutexLock mu(Thread::Current(), *Locks::thread_list_lock_);
ThreadList* const thread_list = Runtime::Current()->GetThreadList();
Thread* expected_owner = thread_list->FindThreadByThreadId(expected_owner_thread_id);
Thread* found_owner = thread_list->FindThreadByThreadId(found_owner_thread_id);
// Re-read owner now that we hold lock.
Thread* current_owner = (monitor != nullptr) ? monitor->GetOwner() : nullptr;
if (current_owner != nullptr) {
current_owner_thread_id = current_owner->GetThreadId();
}
// Get short descriptions of the threads involved.
current_owner_string = ThreadToString(current_owner);
expected_owner_string = expected_owner != nullptr ? ThreadToString(expected_owner) : "unnamed";
found_owner_string = found_owner != nullptr ? ThreadToString(found_owner) : "unnamed";
}
if (current_owner_thread_id == 0u) {
if (found_owner_thread_id == 0u) {
ThrowIllegalMonitorStateExceptionF("unlock of unowned monitor on object of type '%s'"
" on thread '%s'",
mirror::Object::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(),
mirror::Object::PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
}
} else {
if (found_owner_thread_id == 0u) {
// 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(),
mirror::Object::PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
} else {
if (found_owner_thread_id != current_owner_thread_id) {
// 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(),
mirror::Object::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(),
mirror::Object::PrettyTypeOf(o).c_str(),
expected_owner_string.c_str());
}
}
}
}
bool Monitor::Unlock(Thread* self) {
DCHECK(self != nullptr);
uint32_t owner_thread_id = 0u;
DCHECK(!monitor_lock_.IsExclusiveHeld(self));
monitor_lock_.Lock(self);
Thread* owner = owner_;
if (owner != nullptr) {
owner_thread_id = owner->GetThreadId();
}
if (owner == self) {
// We own the monitor, so nobody else can be in here.
AtraceMonitorUnlock();
if (lock_count_ == 0) {
owner_ = nullptr;
locking_method_ = nullptr;
locking_dex_pc_ = 0;
SignalContendersAndReleaseMonitorLock(self);
return true;
} else {
--lock_count_;
monitor_lock_.Unlock(self);
return true;
}
}
// 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->GetThreadId(), owner_thread_id, this);
monitor_lock_.Unlock(self);
return false;
}
void Monitor::SignalContendersAndReleaseMonitorLock(Thread* self) {
// We want to signal one thread to wake up, to acquire the monitor that
// we are releasing. This could either be a Thread waiting on its own
// ConditionVariable, or a thread waiting on monitor_contenders_.
while (wake_set_ != nullptr) {
// No risk of waking ourselves here; since monitor_lock_ is not released until we're ready to
// return, notify can't move the current thread from wait_set_ to wake_set_ until this
// method is done checking wake_set_.
Thread* thread = wake_set_;
wake_set_ = thread->GetWaitNext();
thread->SetWaitNext(nullptr);
// Check to see if the thread is still waiting.
{
// In the case of wait(), we'll be acquiring another thread's GetWaitMutex with
// self's GetWaitMutex held. This does not risk deadlock, because we only acquire this lock
// for threads in the wake_set_. A thread can only enter wake_set_ from Notify or NotifyAll,
// and those hold monitor_lock_. Thus, the threads whose wait mutexes we acquire here must
// have already been released from wait(), since we have not released monitor_lock_ until
// after we've chosen our thread to wake, so there is no risk of the following lock ordering
// leading to deadlock:
// Thread 1 waits
// Thread 2 waits
// Thread 3 moves threads 1 and 2 from wait_set_ to wake_set_
// Thread 1 enters this block, and attempts to acquire Thread 2's GetWaitMutex to wake it
// Thread 2 enters this block, and attempts to acquire Thread 1's GetWaitMutex to wake it
//
// Since monitor_lock_ is not released until the thread-to-be-woken-up's GetWaitMutex is
// acquired, two threads cannot attempt to acquire each other's GetWaitMutex while holding
// their own and cause deadlock.
MutexLock wait_mu(self, *thread->GetWaitMutex());
if (thread->GetWaitMonitor() != nullptr) {
// Release the lock, so that a potentially awakened thread will not
// immediately contend on it. The lock ordering here is:
// monitor_lock_, self->GetWaitMutex, thread->GetWaitMutex
monitor_lock_.Unlock(self);
thread->GetWaitConditionVariable()->Signal(self);
return;
}
}
}
// If we didn't wake any threads that were originally waiting on us,
// wake a contender.
monitor_contenders_.Signal(self);
monitor_lock_.Unlock(self);
}
void Monitor::Wait(Thread* self, int64_t ms, int32_t ns,
bool interruptShouldThrow, ThreadState why) {
DCHECK(self != nullptr);
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);
self->ThrowNewExceptionF("Ljava/lang/IllegalArgumentException;",
"timeout arguments out of range: ms=%" PRId64 " ns=%d", ms, ns);
return;
}
/*
* 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.
*/
int prev_lock_count = lock_count_;
lock_count_ = 0;
owner_ = nullptr;
ArtMethod* saved_method = locking_method_;
locking_method_ = nullptr;
uintptr_t saved_dex_pc = locking_dex_pc_;
locking_dex_pc_ = 0;
AtraceMonitorUnlock(); // For the implict Unlock() just above. This will only end the deepest
// nesting, but that is enough for the visualization, and corresponds to
// the single Lock() we do afterwards.
AtraceMonitorLock(self, GetObject(), /* is_wait= */ true);
bool was_interrupted = false;
bool timed_out = false;
{
// 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.
ScopedThreadSuspension sts(self, why);
// Pseudo-atomically wait on self's wait_cond_ and release the monitor lock.
MutexLock mu(self, *self->GetWaitMutex());
/*
* Add ourselves to the set of threads waiting on this monitor.
* It's important that we are only added to the wait set after
* acquiring our GetWaitMutex, so that calls to Notify() that occur after we
* have released monitor_lock_ will not move us from wait_set_ to wake_set_
* until we've signalled contenders on this monitor.
*/
AppendToWaitSet(self);
++num_waiters_;
// 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.
SignalContendersAndReleaseMonitorLock(self);
// Handle the case where the thread was interrupted before we called wait().
if (self->IsInterrupted()) {
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;
timed_out = self->GetWaitConditionVariable()->TimedWait(self, ms, ns);
}
was_interrupted = self->IsInterrupted();
}
}
{
// 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);
}
// Allocate the interrupted exception not holding the monitor lock since it may cause a GC.
// If the GC requires acquiring the monitor for enqueuing cleared references, this would
// cause a deadlock if the monitor is held.
if (was_interrupted && interruptShouldThrow) {
/*
* 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."
*/
self->SetInterrupted(false);
self->ThrowNewException("Ljava/lang/InterruptedException;", nullptr);
}
AtraceMonitorUnlock(); // End Wait().
// We just slept, tell the runtime callbacks about this.
Runtime::Current()->GetRuntimeCallbacks()->MonitorWaitFinished(this, timed_out);
// Re-acquire the monitor and lock.
Lock<LockReason::kForWait>(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);
}
void Monitor::Notify(Thread* self) {
DCHECK(self != nullptr);
MutexLock mu(self, monitor_lock_);
// Make sure that we hold the lock.
if (owner_ != self) {
ThrowIllegalMonitorStateExceptionF("object not locked by thread before notify()");
return;
}
// Move one thread from waiters to wake set
Thread* to_move = wait_set_;
if (to_move != nullptr) {
wait_set_ = to_move->GetWaitNext();
to_move->SetWaitNext(wake_set_);
wake_set_ = to_move;
}
}
void Monitor::NotifyAll(Thread* self) {
DCHECK(self != nullptr);
MutexLock mu(self, monitor_lock_);
// Make sure that we hold the lock.
if (owner_ != self) {
ThrowIllegalMonitorStateExceptionF("object not locked by thread before notifyAll()");
return;
}
// Move all threads from waiters to wake set
Thread* to_move = wait_set_;
if (to_move != nullptr) {
wait_set_ = nullptr;
Thread* move_to = wake_set_;
if (move_to == nullptr) {
wake_set_ = to_move;
return;
}
while (move_to->GetWaitNext() != nullptr) {
move_to = move_to->GetWaitNext();
}
move_to->SetWaitNext(to_move);
}
}
bool Monitor::Deflate(Thread* self, ObjPtr<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 (static_cast<uint32_t>(monitor->lock_count_) > LockWord::kThinLockMaxCount) {
return false;
}
// Deflate to a thin lock.
LockWord new_lw = LockWord::FromThinLockId(owner->GetThreadId(),
monitor->lock_count_,
lw.GCState());
// Assume no concurrent read barrier state changes as mutators are suspended.
obj->SetLockWord(new_lw, false);
VLOG(monitor) << "Deflated " << obj << " to thin lock " << owner->GetTid() << " / "
<< monitor->lock_count_;
} else if (monitor->HasHashCode()) {
LockWord new_lw = LockWord::FromHashCode(monitor->GetHashCode(), lw.GCState());
// Assume no concurrent read barrier state changes as mutators are suspended.
obj->SetLockWord(new_lw, 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.
LockWord new_lw = LockWord::FromDefault(lw.GCState());
// Assume no concurrent read barrier state changes as mutators are suspended.
obj->SetLockWord(new_lw, false);
VLOG(monitor) << "Deflated" << obj << " to empty lock word";
}
// The monitor is deflated, mark the object as null 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, ObjPtr<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;
{
ScopedThreadSuspension sts(self, kWaitingForLockInflation);
owner = thread_list->SuspendThreadByThreadId(owner_thread_id,
SuspendReason::kInternal,
&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);
}
bool resumed = thread_list->Resume(owner, SuspendReason::kInternal);
DCHECK(resumed);
}
self->SetMonitorEnterObject(nullptr);
}
}
// Fool annotalysis into thinking that the lock on obj is acquired.
static ObjPtr<mirror::Object> FakeLock(ObjPtr<mirror::Object> obj)
EXCLUSIVE_LOCK_FUNCTION(obj.Ptr()) NO_THREAD_SAFETY_ANALYSIS {
return obj;
}
// Fool annotalysis into thinking that the lock on obj is release.
static ObjPtr<mirror::Object> FakeUnlock(ObjPtr<mirror::Object> obj)
UNLOCK_FUNCTION(obj.Ptr()) NO_THREAD_SAFETY_ANALYSIS {
return obj;
}
ObjPtr<mirror::Object> Monitor::MonitorEnter(Thread* self,
ObjPtr<mirror::Object> obj,
bool trylock) {
DCHECK(self != nullptr);
DCHECK(obj != nullptr);
self->AssertThreadSuspensionIsAllowable();
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) {
// We initially read the lockword with ordinary Java/relaxed semantics. When stronger
// semantics are needed, we address it below. Since GetLockWord bottoms out to a relaxed load,
// we can fix it later, in an infrequently executed case, with a fence.
LockWord lock_word = h_obj->GetLockWord(false);
switch (lock_word.GetState()) {
case LockWord::kUnlocked: {
// No ordering required for preceding lockword read, since we retest.
LockWord thin_locked(LockWord::FromThinLockId(thread_id, 0, lock_word.GCState()));
if (h_obj->CasLockWord(lock_word, thin_locked, CASMode::kWeak, std::memory_order_acquire)) {
AtraceMonitorLock(self, h_obj.Get(), /* is_wait= */ false);
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) {
// No ordering required for initial lockword read.
// 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,
lock_word.GCState()));
// Only this thread pays attention to the count. Thus there is no need for stronger
// than relaxed memory ordering.
if (!kUseReadBarrier) {
h_obj->SetLockWord(thin_locked, /* as_volatile= */ false);
AtraceMonitorLock(self, h_obj.Get(), /* is_wait= */ false);
return h_obj.Get(); // Success!
} else {
// Use CAS to preserve the read barrier state.
if (h_obj->CasLockWord(lock_word,
thin_locked,
CASMode::kWeak,
std::memory_order_relaxed)) {
AtraceMonitorLock(self, h_obj.Get(), /* is_wait= */ false);
return h_obj.Get(); // Success!
}
}
continue; // Go again.
} else {
// We'd overflow the recursion count, so inflate the monitor.
InflateThinLocked(self, h_obj, lock_word, 0);
}
} else {
if (trylock) {
return nullptr;
}
// Contention.
contention_count++;
Runtime* runtime = Runtime::Current();
if (contention_count <= runtime->GetMaxSpinsBeforeThinLockInflation()) {
// TODO: Consider switching the thread state to kWaitingForLockInflation 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.
// TODO: We should literally spin first, without sched_yield. Sched_yield either does
// nothing (at significant expense), or guarantees that we wait at least microseconds.
// If the owner is running, I would expect the median lock hold time to be hundreds
// of nanoseconds or less.
sched_yield();
} else {
contention_count = 0;
// No ordering required for initial lockword read. Install rereads it anyway.
InflateThinLocked(self, h_obj, lock_word, 0);
}
}
continue; // Start from the beginning.
}
case LockWord::kFatLocked: {
// We should have done an acquire read of the lockword initially, to ensure
// visibility of the monitor data structure. Use an explicit fence instead.
std::atomic_thread_fence(std::memory_order_acquire);
Monitor* mon = lock_word.FatLockMonitor();
if (trylock) {
return mon->TryLock(self) ? h_obj.Get() : nullptr;
} else {
mon->Lock(self);
return h_obj.Get(); // Success!
}
}
case LockWord::kHashCode:
// Inflate with the existing hashcode.
// Again no ordering required for initial lockword read, since we don't rely
// on the visibility of any prior computation.
Inflate(self, nullptr, h_obj.Get(), lock_word.GetHashCode());
continue; // Start from the beginning.
default: {
LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
UNREACHABLE();
}
}
}
}
bool Monitor::MonitorExit(Thread* self, ObjPtr<mirror::Object> obj) {
DCHECK(self != nullptr);
DCHECK(obj != nullptr);
self->AssertThreadSuspensionIsAllowable();
obj = FakeUnlock(obj);
StackHandleScope<1> hs(self);
Handle<mirror::Object> h_obj(hs.NewHandle(obj));
while (true) {
LockWord lock_word = obj->GetLockWord(true);
switch (lock_word.GetState()) {
case LockWord::kHashCode:
// Fall-through.
case LockWord::kUnlocked:
FailedUnlock(h_obj.Get(), self->GetThreadId(), 0u, 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) {
FailedUnlock(h_obj.Get(), thread_id, owner_thread_id, nullptr);
return false; // Failure.
} else {
// We own the lock, decrease the recursion count.
LockWord new_lw = LockWord::Default();
if (lock_word.ThinLockCount() != 0) {
uint32_t new_count = lock_word.ThinLockCount() - 1;
new_lw = LockWord::FromThinLockId(thread_id, new_count, lock_word.GCState());
} else {
new_lw = LockWord::FromDefault(lock_word.GCState());
}
if (!kUseReadBarrier) {
DCHECK_EQ(new_lw.ReadBarrierState(), 0U);
// TODO: This really only needs memory_order_release, but we currently have
// no way to specify that. In fact there seem to be no legitimate uses of SetLockWord
// with a final argument of true. This slows down x86 and ARMv7, but probably not v8.
h_obj->SetLockWord(new_lw, true);
AtraceMonitorUnlock();
// Success!
return true;
} else {
// Use CAS to preserve the read barrier state.
if (h_obj->CasLockWord(lock_word, new_lw, CASMode::kWeak, std::memory_order_release)) {
AtraceMonitorUnlock();
// Success!
return true;
}
}
continue; // Go again.
}
}
case LockWord::kFatLocked: {
Monitor* mon = lock_word.FatLockMonitor();
return mon->Unlock(self);
}
default: {
LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
UNREACHABLE();
}
}
}
}
void Monitor::Wait(Thread* self,
ObjPtr<mirror::Object> obj,
int64_t ms,
int32_t ns,
bool interruptShouldThrow,
ThreadState why) {
DCHECK(self != nullptr);
DCHECK(obj != nullptr);
StackHandleScope<1> hs(self);
Handle<mirror::Object> h_obj(hs.NewHandle(obj));
Runtime::Current()->GetRuntimeCallbacks()->ObjectWaitStart(h_obj, ms);
if (UNLIKELY(self->ObserveAsyncException() || self->IsExceptionPending())) {
// See b/65558434 for information on handling of exceptions here.
return;
}
LockWord lock_word = h_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, h_obj.Get(), 0);
lock_word = h_obj->GetLockWord(true);
}
break;
}
case LockWord::kFatLocked: // Unreachable given the loop condition above. Fall-through.
default: {
LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
UNREACHABLE();
}
}
}
Monitor* mon = lock_word.FatLockMonitor();
mon->Wait(self, ms, ns, interruptShouldThrow, why);
}
void Monitor::DoNotify(Thread* self, ObjPtr<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();
UNREACHABLE();
}
}
}
uint32_t Monitor::GetLockOwnerThreadId(ObjPtr<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";
UNREACHABLE();
}
}
}
ThreadState Monitor::FetchState(const Thread* thread,
/* out */ ObjPtr<mirror::Object>* monitor_object,
/* out */ uint32_t* lock_owner_tid) {
DCHECK(monitor_object != nullptr);
DCHECK(lock_owner_tid != nullptr);
*monitor_object = nullptr;
*lock_owner_tid = ThreadList::kInvalidThreadId;
ThreadState state = thread->GetState();
switch (state) {
case kWaiting:
case kTimedWaiting:
case kSleeping:
{
Thread* self = Thread::Current();
MutexLock mu(self, *thread->GetWaitMutex());
Monitor* monitor = thread->GetWaitMonitor();
if (monitor != nullptr) {
*monitor_object = monitor->GetObject();
}
}
break;
case kBlocked:
case kWaitingForLockInflation:
{
ObjPtr<mirror::Object> lock_object = thread->GetMonitorEnterObject();
if (lock_object != nullptr) {
if (kUseReadBarrier && Thread::Current()->GetIsGcMarking()) {
// We may call Thread::Dump() in the middle of the CC thread flip and this thread's stack
// may have not been flipped yet and "pretty_object" may be a from-space (stale) ref, in
// which case the GetLockOwnerThreadId() call below will crash. So explicitly mark/forward
// it here.
lock_object = ReadBarrier::Mark(lock_object.Ptr());
}
*monitor_object = lock_object;
*lock_owner_tid = lock_object->GetLockOwnerThreadId();
}
}
break;
default:
break;
}
return state;
}
ObjPtr<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...
ObjPtr<mirror::Object> result = thread->GetMonitorEnterObject();
if (result == nullptr) {
// ...but also a monitor that the thread is waiting on.
MutexLock mu(Thread::Current(), *thread->GetWaitMutex());
Monitor* monitor = thread->GetWaitMonitor();
if (monitor != nullptr) {
result = monitor->GetObject();
}
}
return result;
}
void Monitor::VisitLocks(StackVisitor* stack_visitor,
void (*callback)(ObjPtr<mirror::Object>, void*),
void* callback_context,
bool abort_on_failure) {
ArtMethod* m = stack_visitor->GetMethod();
CHECK(m != nullptr);
// 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()) {
ObjPtr<mirror::Object> jni_this =
stack_visitor->GetCurrentHandleScope(sizeof(void*))->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?
CHECK(m->GetCodeItem() != nullptr) << m->PrettyMethod();
CodeItemDataAccessor accessor(m->DexInstructionData());
if (accessor.TriesSize() == 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 == dex::kDexNoIndex) {
LOG(ERROR) << "Could not find dex_pc for " << m->PrettyMethod();
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<verifier::MethodVerifier::DexLockInfo> monitor_enter_dex_pcs;
verifier::MethodVerifier::FindLocksAtDexPc(m,
dex_pc,
&monitor_enter_dex_pcs,
Runtime::Current()->GetTargetSdkVersion());
for (verifier::MethodVerifier::DexLockInfo& dex_lock_info : monitor_enter_dex_pcs) {
// As a debug check, check that dex PC corresponds to a monitor-enter.
if (kIsDebugBuild) {
const Instruction& monitor_enter_instruction = accessor.InstructionAt(dex_lock_info.dex_pc);
CHECK_EQ(monitor_enter_instruction.Opcode(), Instruction::MONITOR_ENTER)
<< "expected monitor-enter @" << dex_lock_info.dex_pc << "; was "
<< reinterpret_cast<const void*>(&monitor_enter_instruction);
}
// Iterate through the set of dex registers, as the compiler may not have held all of them
// live.
bool success = false;
for (uint32_t dex_reg : dex_lock_info.dex_registers) {
uint32_t value;
success = stack_visitor->GetVReg(m, dex_reg, kReferenceVReg, &value);
if (success) {
ObjPtr<mirror::Object> o = reinterpret_cast<mirror::Object*>(value);
callback(o, callback_context);
break;
}
}
DCHECK(success) << "Failed to find/read reference for monitor-enter at dex pc "
<< dex_lock_info.dex_pc
<< " in method "
<< m->PrettyMethod();
if (!success) {
LOG(WARNING) << "Had a lock reported for dex pc " << dex_lock_info.dex_pc
<< " but was not able to fetch a corresponding object!";
}
}
}
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";
UNREACHABLE();
}
}
bool Monitor::IsLocked() REQUIRES_SHARED(Locks::mutator_lock_) {
MutexLock mu(Thread::Current(), monitor_lock_);
return owner_ != nullptr;
}
void Monitor::TranslateLocation(ArtMethod* method,
uint32_t dex_pc,
const char** source_file,
int32_t* line_number) {
// If method is null, location is unknown
if (method == nullptr) {
*source_file = "";
*line_number = 0;
return;
}
*source_file = method->GetDeclaringClassSourceFile();
if (*source_file == nullptr) {
*source_file = "";
}
*line_number = method->GetLineNumFromDexPC(dex_pc);
}
uint32_t Monitor::GetOwnerThreadId() {
MutexLock mu(Thread::Current(), monitor_lock_);
Thread* owner = owner_;
if (owner != nullptr) {
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() {
CHECK(!kUseReadBarrier);
MutexLock mu(Thread::Current(), monitor_list_lock_);
allow_new_monitors_ = false;
}
void MonitorList::AllowNewMonitors() {
CHECK(!kUseReadBarrier);
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
allow_new_monitors_ = true;
monitor_add_condition_.Broadcast(self);
}
void MonitorList::BroadcastForNewMonitors() {
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
monitor_add_condition_.Broadcast(self);
}
void MonitorList::Add(Monitor* m) {
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
// CMS needs this to block for concurrent reference processing because an object allocated during
// the GC won't be marked and concurrent reference processing would incorrectly clear the JNI weak
// ref. But CC (kUseReadBarrier == true) doesn't because of the to-space invariant.
while (!kUseReadBarrier && UNLIKELY(!allow_new_monitors_)) {
// Check and run the empty checkpoint before blocking so the empty checkpoint will work in the
// presence of threads blocking for weak ref access.
self->CheckEmptyCheckpointFromWeakRefAccess(&monitor_list_lock_);
monitor_add_condition_.WaitHoldingLocks(self);
}
list_.push_front(m);
}
void MonitorList::SweepMonitorList(IsMarkedVisitor* visitor) {
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.
ObjPtr<mirror::Object> obj = m->GetObject<kWithoutReadBarrier>();
// The object of a monitor can be null if we have deflated it.
ObjPtr<mirror::Object> new_obj = obj != nullptr ? visitor->IsMarked(obj.Ptr()) : 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;
}
}
}
size_t MonitorList::Size() {
Thread* self = Thread::Current();
MutexLock mu(self, monitor_list_lock_);
return list_.size();
}
class MonitorDeflateVisitor : public IsMarkedVisitor {
public:
MonitorDeflateVisitor() : self_(Thread::Current()), deflate_count_(0) {}
mirror::Object* IsMarked(mirror::Object* object) override
REQUIRES_SHARED(Locks::mutator_lock_) {
if (Monitor::Deflate(self_, object)) {
DCHECK_NE(object->GetLockWord(true).GetState(), LockWord::kFatLocked);
++deflate_count_;
// If we deflated, return null so that the monitor gets removed from the array.
return nullptr;
}
return object; // Monitor was not deflated.
}
Thread* const self_;
size_t deflate_count_;
};
size_t MonitorList::DeflateMonitors() {
MonitorDeflateVisitor visitor;
Locks::mutator_lock_->AssertExclusiveHeld(visitor.self_);
SweepMonitorList(&visitor);
return visitor.deflate_count_;
}
MonitorInfo::MonitorInfo(ObjPtr<mirror::Object> obj) : owner_(nullptr), 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());
DCHECK(owner_ != nullptr) << "Thin-locked without owner!";
entry_count_ = 1 + lock_word.ThinLockCount();
// Thin locks have no waiters.
break;
case LockWord::kFatLocked: {
Monitor* mon = lock_word.FatLockMonitor();
owner_ = mon->owner_;
// Here it is okay for the owner to be null since we don't reset the LockWord back to
// kUnlocked until we get a GC. In cases where this hasn't happened yet we will have a fat
// lock without an owner.
if (owner_ != nullptr) {
entry_count_ = 1 + mon->lock_count_;
} else {
DCHECK_EQ(mon->lock_count_, 0) << "Monitor is fat-locked without any owner!";
}
for (Thread* waiter = mon->wait_set_; waiter != nullptr; waiter = waiter->GetWaitNext()) {
waiters_.push_back(waiter);
}
break;
}
}
}
} // namespace art