runtime/vm/heap/safepoint.cc - sdk - Git at Google

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.

 #include "vm/heap/safepoint.h"

 #include "vm/heap/heap.h"
 #include "vm/thread.h"
 #include "vm/thread_registry.h"

 namespace dart {

 DEFINE_FLAG(bool, trace_safepoint, false, "Trace Safepoint logic.");

 SafepointOperationScope::SafepointOperationScope(Thread* T,
                                                  SafepointLevel level)
     : ThreadStackResource(T), level_(level) {
   ASSERT(T != nullptr && T->isolate_group() != nullptr);

   auto handler = T->isolate_group()->safepoint_handler();
   handler->SafepointThreads(T, level_);
 }

 SafepointOperationScope::~SafepointOperationScope() {
   Thread* T = thread();
   ASSERT(T != nullptr && T->isolate_group() != nullptr);

   auto handler = T->isolate_group()->safepoint_handler();
   handler->ResumeThreads(T, level_);
 }

 ForceGrowthSafepointOperationScope::ForceGrowthSafepointOperationScope(
     Thread* T,
     SafepointLevel level)
     : ThreadStackResource(T), level_(level) {
   ASSERT(T != nullptr);
   IsolateGroup* IG = T->isolate_group();
   ASSERT(IG != nullptr);

   T->IncrementForceGrowthScopeDepth();

   auto handler = IG->safepoint_handler();
   handler->SafepointThreads(T, level_);
 }

 ForceGrowthSafepointOperationScope::~ForceGrowthSafepointOperationScope() {
   Thread* T = thread();
   ASSERT(T != nullptr);
   IsolateGroup* IG = T->isolate_group();
   ASSERT(IG != nullptr);

   auto handler = IG->safepoint_handler();
   handler->ResumeThreads(T, level_);

   T->DecrementForceGrowthScopeDepth();
   if (!T->force_growth()) {
     // Check if we passed the growth limit during the scope.
     T->heap()->CheckCatchUp(T);
   }
 }

 SafepointHandler::SafepointHandler(IsolateGroup* isolate_group)
     : isolate_group_(isolate_group) {
   handlers_[SafepointLevel::kGC] =
       new LevelHandler(isolate_group, SafepointLevel::kGC);
   handlers_[SafepointLevel::kGCAndDeopt] =
       new LevelHandler(isolate_group, SafepointLevel::kGCAndDeopt);
   handlers_[SafepointLevel::kGCAndDeoptAndReload] =
       new LevelHandler(isolate_group, SafepointLevel::kGCAndDeoptAndReload);
 }

 SafepointHandler::~SafepointHandler() {
   for (intptr_t level = 0; level < SafepointLevel::kNumLevels; ++level) {
     ASSERT(handlers_[level]->owner_ == nullptr);
     delete handlers_[level];
   }
 }

 void SafepointHandler::SafepointThreads(Thread* T, SafepointLevel level) {
   ASSERT(T->no_safepoint_scope_depth() == 0);
   ASSERT(T->execution_state() == Thread::kThreadInVM);
   ASSERT(T->current_safepoint_level() >= level);

   MallocGrowableArray<Dart_Port> oob_isolates;
   {
     MonitorLocker tl(threads_lock());

     // Allow recursive deopt safepoint operation.
     if (handlers_[level]->owner_ == T) {
       // If we own this safepoint level already we have to own the lower levels
       // as well.
       AssertWeOwnLowerLevelSafepoints(T, level);

       for (intptr_t i = 0; i <= level; ++i) {
         handlers_[i]->operation_count_++;
       }
       return;
     }

     // This level of nesting is not allowed (this thread cannot own lower levels
     // and then later try acquire higher levels).
     AssertWeDoNotOwnLowerLevelSafepoints(T, level);

     // Mark this thread at safepoint and possibly notify waiting threads.
     {
       MonitorLocker tl(T->thread_lock());
       // We only enter [level] here. That means a higher level that is waiting
       // for us to check-in will not consider us as not parked. This is required
       // since we are not actually parked (we can finish running this method and
       // then caller continues).
       EnterSafepointLocked(T, &tl, level);
     }

     // Wait until other safepoint operations are done & mark us as owning
     // the safepoint - so no other thread can.
     while (handlers_[level]->SafepointInProgress()) {
       tl.Wait();
     }
     handlers_[level]->SetSafepointInProgress(T);

     // Ensure a thread is at a safepoint or notify it to get to one.
     handlers_[level]->NotifyThreadsToGetToSafepointLevel(T, &oob_isolates);
   }

   for (auto main_port : oob_isolates) {
     Isolate::SendInternalLibMessage(main_port, Isolate::kCheckForReload,
                                     /*ignored=*/-1);
   }

   // Now wait for all threads that are not already at a safepoint to check-in.
   handlers_[level]->WaitUntilThreadsReachedSafepointLevel();

   // No other mutator is running at this point. We'll set ourselves as owners of
   // all the lower levels as well - since higher levels provide even more
   // guarantees that lower levels (e.g. others being stopped at places where
   // one can deopt also implies one can gc)
   AcquireLowerLevelSafepoints(T, level);

   // The current thread owns the safepoint, but it will continue to run and as
   // such is not at any "point" that can be considered safe.
   {
     MonitorLocker tl(T->thread_lock());
     ExitSafepointLocked(T, &tl, level);
   }
 }

 void SafepointHandler::AssertWeOwnLowerLevelSafepoints(Thread* T,
                                                        SafepointLevel level) {
   for (intptr_t lower_level = level - 1; lower_level >= 0; --lower_level) {
     RELEASE_ASSERT(handlers_[lower_level]->owner_ == T);
   }
 }

 void SafepointHandler::AssertWeDoNotOwnLowerLevelSafepoints(
     Thread* T,
     SafepointLevel level) {
   for (intptr_t lower_level = level - 1; lower_level >= 0; --lower_level) {
     RELEASE_ASSERT(handlers_[lower_level]->owner_ != T);
   }
 }

 void SafepointHandler::LevelHandler::NotifyThreadsToGetToSafepointLevel(
     Thread* T,
     MallocGrowableArray<Dart_Port>* oob_isolates) {
   ASSERT(num_threads_not_parked_ == 0);
   for (auto current = isolate_group()->thread_registry()->active_list();
        current != nullptr; current = current->next()) {
     MonitorLocker tl(current->thread_lock());
     if (!current->BypassSafepoints() && current != T) {
       const uint32_t state = current->SetSafepointRequested(level_, true);
       if (!Thread::IsAtSafepoint(level_, state)) {
         if (level_ == SafepointLevel::kGCAndDeoptAndReload) {
           // Interrupt the mutator by sending an reload OOB message. The
           // mutator will only check-in once it's handling the reload OOB
           // message.
           //
           // If there's no isolate, it may be a helper thread that has entered
           // via `Thread::EnterIsolateGroupAsHelper()`. In that case we cannot
           // send an OOB message. Instead we'll have to wait until that thread
           // de-schedules itself.
           auto isolate = current->scheduled_dart_mutator_isolate();
           if (isolate != nullptr) {
             oob_isolates->Add(isolate->main_port());
           }
         } else {
           // Interrupt the mutator and ask it to block at any interruption
           // point.
           if (current->IsDartMutatorThread()) {
             current->ScheduleInterrupts(Thread::kVMInterrupt);
           }
         }
         MonitorLocker sl(&parked_lock_);
         num_threads_not_parked_++;
       }
     }
   }
 }

 void SafepointHandler::ResumeThreads(Thread* T, SafepointLevel level) {
   {
     MonitorLocker sl(threads_lock());

     ASSERT(handlers_[level]->SafepointInProgress());
     ASSERT(handlers_[level]->owner_ == T);
     AssertWeOwnLowerLevelSafepoints(T, level);

     // We allow recursive safepoints.
     if (handlers_[level]->operation_count_ > 1) {
       for (intptr_t i = 0; i <= level; ++i) {
         handlers_[i]->operation_count_--;
       }
       return;
     }

     ReleaseLowerLevelSafepoints(T, level);
     handlers_[level]->ResetSafepointInProgress(T);
     handlers_[level]->NotifyThreadsToContinue(T);
     sl.NotifyAll();
   }
 }

 void SafepointHandler::LevelHandler::WaitUntilThreadsReachedSafepointLevel() {
   MonitorLocker sl(&parked_lock_);
   intptr_t num_attempts = 0;
   while (num_threads_not_parked_ > 0) {
     Monitor::WaitResult retval = sl.Wait(1000);
     if (retval == Monitor::kTimedOut) {
       num_attempts += 1;
       if (FLAG_trace_safepoint && num_attempts > 10) {
         for (auto current = isolate_group()->thread_registry()->active_list();
              current != nullptr; current = current->next()) {
           if (!current->IsAtSafepoint(level_)) {
             OS::PrintErr("Attempt:%" Pd " waiting for thread %s to check in\n",
                          num_attempts, current->os_thread()->name());
           }
         }
       }
     }
   }
 }

 void SafepointHandler::AcquireLowerLevelSafepoints(Thread* T,
                                                    SafepointLevel level) {
   MonitorLocker tl(threads_lock());
   ASSERT(handlers_[level]->owner_ == T);
   for (intptr_t lower_level = level - 1; lower_level >= 0; --lower_level) {
     ASSERT(!handlers_[lower_level]->SafepointInProgress());
     handlers_[lower_level]->SetSafepointInProgress(T);
     ASSERT(handlers_[lower_level]->owner_ == T);
   }
 }

 void SafepointHandler::ReleaseLowerLevelSafepoints(Thread* T,
                                                    SafepointLevel level) {
   for (intptr_t lower_level = 0; lower_level < level; ++lower_level) {
     handlers_[lower_level]->ResetSafepointInProgress(T);
   }
 }

 void SafepointHandler::LevelHandler::NotifyThreadsToContinue(Thread* T) {
   for (auto current = isolate_group()->thread_registry()->active_list();
        current != nullptr; current = current->next()) {
     MonitorLocker tl(current->thread_lock());
     if (!current->BypassSafepoints() && current != T) {
       bool resume = false;
       for (intptr_t lower_level = level_; lower_level >= 0; --lower_level) {
         if (Thread::IsBlockedForSafepoint(current->SetSafepointRequested(
                 static_cast<SafepointLevel>(lower_level), false))) {
           resume = true;
         }
       }
       if (resume) {
         tl.Notify();
       }
     }
   }
 }

 void SafepointHandler::EnterSafepointUsingLock(Thread* T) {
   MonitorLocker tl(T->thread_lock());
   EnterSafepointLocked(T, &tl, T->current_safepoint_level());
 }

 void SafepointHandler::ExitSafepointUsingLock(Thread* T) {
   MonitorLocker tl(T->thread_lock());
   ASSERT(T->IsAtSafepoint());
   ExitSafepointLocked(T, &tl, T->current_safepoint_level());
   ASSERT(!T->IsSafepointRequestedLocked(T->current_safepoint_level()));
 }

 SafepointLevel SafepointHandler::InnermostSafepointOperation(
     const Thread* current_thread) const {
   // The [current_thread] may not own the active safepoint.
   intptr_t last_count = -1;
   SafepointLevel last_level = SafepointLevel::kNoSafepoint;

   // Notice: We access SafepointLevel::{owner_,operation_count_} fields
   // without lock. This is ok since:
   //
   //   * If the current thread is the owner, then it will be the one that has
   //     last written `Thread::Current()` to the `owner_` field (as well as
   //     updated the `operation_count_`) - nobody else can update those fields
   //     in the meantime. Once the current thread exits we set it to `nullptr`.
   //
   //   * If there's no owner or another thread is the owner the value cannot be
   //     `Thread::Current()`: only our thread can write that particular value.
   //
   //   => Even if there's racy writes by another thread, the logic is still
   //      safe.
   //
   for (intptr_t level = 0; level < SafepointLevel::kNumLevels; ++level) {
     if (handlers_[level]->owner_ == current_thread) {
       const intptr_t count = handlers_[level]->operation_count_;
       if (count < last_count) return last_level;
       last_count = count;
       last_level = static_cast<SafepointLevel>(level);
     } else {
       return last_level;
     }
   }
   return last_level;
 }

 void SafepointHandler::BlockForSafepoint(Thread* T) {
   ASSERT(!T->BypassSafepoints());
   MonitorLocker tl(T->thread_lock());
   // This takes into account the safepoint level the thread can participate in.
   const SafepointLevel level = T->current_safepoint_level();
   if (T->IsSafepointRequestedLocked(level)) {
     EnterSafepointLocked(T, &tl, level);
     ExitSafepointLocked(T, &tl, level);
     ASSERT(!T->IsSafepointRequestedLocked(level));
   }
 }

 void SafepointHandler::EnterSafepointLocked(Thread* T,
                                             MonitorLocker* tl,
                                             SafepointLevel level) {
   T->SetAtSafepoint(true, level);
   // Several safepointing operations (at different) levels may happen at same
   // time. Ensure we notify all of them that we are parked now.
   for (intptr_t i = 0; i <= level; ++i) {
     if (T->IsSafepointLevelRequestedLocked(static_cast<SafepointLevel>(i))) {
       handlers_[i]->NotifyWeAreParked(T);
     }
   }
 }

 void SafepointHandler::LevelHandler::NotifyWeAreParked(Thread* T) {
   ASSERT(owner_ != nullptr);
   MonitorLocker sl(&parked_lock_);
   ASSERT(num_threads_not_parked_ > 0);
   num_threads_not_parked_ -= 1;
   if (num_threads_not_parked_ == 0) {
     sl.Notify();
   }
 }

 void SafepointHandler::ExitSafepointLocked(Thread* T,
                                            MonitorLocker* tl,
                                            SafepointLevel level) {
   while (T->IsSafepointRequestedLocked(level)) {
     T->SetBlockedForSafepoint(true);
     tl->Wait();
     T->SetBlockedForSafepoint(false);
   }
   T->SetAtSafepoint(false, level);
 }

 }  // namespace dart
	// Copyright (c) 2016, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.

	#include "vm/heap/safepoint.h"

	#include "vm/heap/heap.h"
	#include "vm/thread.h"
	#include "vm/thread_registry.h"

	namespace dart {

	DEFINE_FLAG(bool, trace_safepoint, false, "Trace Safepoint logic.");

	SafepointOperationScope::SafepointOperationScope(Thread* T,
	SafepointLevel level)
	: ThreadStackResource(T), level_(level) {
	ASSERT(T != nullptr && T->isolate_group() != nullptr);

	auto handler = T->isolate_group()->safepoint_handler();
	handler->SafepointThreads(T, level_);
	}

	SafepointOperationScope::~SafepointOperationScope() {
	Thread* T = thread();
	ASSERT(T != nullptr && T->isolate_group() != nullptr);

	auto handler = T->isolate_group()->safepoint_handler();
	handler->ResumeThreads(T, level_);
	}

	ForceGrowthSafepointOperationScope::ForceGrowthSafepointOperationScope(
	Thread* T,
	SafepointLevel level)
	: ThreadStackResource(T), level_(level) {
	ASSERT(T != nullptr);
	IsolateGroup* IG = T->isolate_group();
	ASSERT(IG != nullptr);

	T->IncrementForceGrowthScopeDepth();

	auto handler = IG->safepoint_handler();
	handler->SafepointThreads(T, level_);
	}

	ForceGrowthSafepointOperationScope::~ForceGrowthSafepointOperationScope() {
	Thread* T = thread();
	ASSERT(T != nullptr);
	IsolateGroup* IG = T->isolate_group();
	ASSERT(IG != nullptr);

	auto handler = IG->safepoint_handler();
	handler->ResumeThreads(T, level_);

	T->DecrementForceGrowthScopeDepth();
	if (!T->force_growth()) {
	// Check if we passed the growth limit during the scope.
	T->heap()->CheckCatchUp(T);
	}
	}

	SafepointHandler::SafepointHandler(IsolateGroup* isolate_group)
	: isolate_group_(isolate_group) {
	handlers_[SafepointLevel::kGC] =
	new LevelHandler(isolate_group, SafepointLevel::kGC);
	handlers_[SafepointLevel::kGCAndDeopt] =
	new LevelHandler(isolate_group, SafepointLevel::kGCAndDeopt);
	handlers_[SafepointLevel::kGCAndDeoptAndReload] =
	new LevelHandler(isolate_group, SafepointLevel::kGCAndDeoptAndReload);
	}

	SafepointHandler::~SafepointHandler() {
	for (intptr_t level = 0; level < SafepointLevel::kNumLevels; ++level) {
	ASSERT(handlers_[level]->owner_ == nullptr);
	delete handlers_[level];
	}
	}

	void SafepointHandler::SafepointThreads(Thread* T, SafepointLevel level) {
	ASSERT(T->no_safepoint_scope_depth() == 0);
	ASSERT(T->execution_state() == Thread::kThreadInVM);
	ASSERT(T->current_safepoint_level() >= level);

	MallocGrowableArray<Dart_Port> oob_isolates;
	{
	MonitorLocker tl(threads_lock());

	// Allow recursive deopt safepoint operation.
	if (handlers_[level]->owner_ == T) {
	// If we own this safepoint level already we have to own the lower levels
	// as well.
	AssertWeOwnLowerLevelSafepoints(T, level);

	for (intptr_t i = 0; i <= level; ++i) {
	handlers_[i]->operation_count_++;
	}
	return;
	}

	// This level of nesting is not allowed (this thread cannot own lower levels
	// and then later try acquire higher levels).
	AssertWeDoNotOwnLowerLevelSafepoints(T, level);

	// Mark this thread at safepoint and possibly notify waiting threads.
	{
	MonitorLocker tl(T->thread_lock());
	// We only enter [level] here. That means a higher level that is waiting
	// for us to check-in will not consider us as not parked. This is required
	// since we are not actually parked (we can finish running this method and
	// then caller continues).
	EnterSafepointLocked(T, &tl, level);
	}

	// Wait until other safepoint operations are done & mark us as owning
	// the safepoint - so no other thread can.
	while (handlers_[level]->SafepointInProgress()) {
	tl.Wait();
	}
	handlers_[level]->SetSafepointInProgress(T);

	// Ensure a thread is at a safepoint or notify it to get to one.
	handlers_[level]->NotifyThreadsToGetToSafepointLevel(T, &oob_isolates);
	}

	for (auto main_port : oob_isolates) {
	Isolate::SendInternalLibMessage(main_port, Isolate::kCheckForReload,
	/ignored=/-1);
	}

	// Now wait for all threads that are not already at a safepoint to check-in.
	handlers_[level]->WaitUntilThreadsReachedSafepointLevel();

	// No other mutator is running at this point. We'll set ourselves as owners of
	// all the lower levels as well - since higher levels provide even more
	// guarantees that lower levels (e.g. others being stopped at places where
	// one can deopt also implies one can gc)
	AcquireLowerLevelSafepoints(T, level);

	// The current thread owns the safepoint, but it will continue to run and as
	// such is not at any "point" that can be considered safe.
	{
	MonitorLocker tl(T->thread_lock());
	ExitSafepointLocked(T, &tl, level);
	}
	}

	void SafepointHandler::AssertWeOwnLowerLevelSafepoints(Thread* T,
	SafepointLevel level) {
	for (intptr_t lower_level = level - 1; lower_level >= 0; --lower_level) {
	RELEASE_ASSERT(handlers_[lower_level]->owner_ == T);
	}
	}

	void SafepointHandler::AssertWeDoNotOwnLowerLevelSafepoints(
	Thread* T,
	SafepointLevel level) {
	for (intptr_t lower_level = level - 1; lower_level >= 0; --lower_level) {
	RELEASE_ASSERT(handlers_[lower_level]->owner_ != T);
	}
	}

	void SafepointHandler::LevelHandler::NotifyThreadsToGetToSafepointLevel(
	Thread* T,
	MallocGrowableArray<Dart_Port>* oob_isolates) {
	ASSERT(num_threads_not_parked_ == 0);
	for (auto current = isolate_group()->thread_registry()->active_list();
	current != nullptr; current = current->next()) {
	MonitorLocker tl(current->thread_lock());
	if (!current->BypassSafepoints() && current != T) {
	const uint32_t state = current->SetSafepointRequested(level_, true);
	if (!Thread::IsAtSafepoint(level_, state)) {
	if (level_ == SafepointLevel::kGCAndDeoptAndReload) {
	// Interrupt the mutator by sending an reload OOB message. The
	// mutator will only check-in once it's handling the reload OOB
	// message.
	//
	// If there's no isolate, it may be a helper thread that has entered
	// via `Thread::EnterIsolateGroupAsHelper()`. In that case we cannot
	// send an OOB message. Instead we'll have to wait until that thread
	// de-schedules itself.
	auto isolate = current->scheduled_dart_mutator_isolate();
	if (isolate != nullptr) {
	oob_isolates->Add(isolate->main_port());
	}
	} else {
	// Interrupt the mutator and ask it to block at any interruption
	// point.
	if (current->IsDartMutatorThread()) {
	current->ScheduleInterrupts(Thread::kVMInterrupt);
	}
	}
	MonitorLocker sl(&parked_lock_);
	num_threads_not_parked_++;
	}
	}
	}
	}

	void SafepointHandler::ResumeThreads(Thread* T, SafepointLevel level) {
	{
	MonitorLocker sl(threads_lock());

	ASSERT(handlers_[level]->SafepointInProgress());
	ASSERT(handlers_[level]->owner_ == T);
	AssertWeOwnLowerLevelSafepoints(T, level);

	// We allow recursive safepoints.
	if (handlers_[level]->operation_count_ > 1) {
	for (intptr_t i = 0; i <= level; ++i) {
	handlers_[i]->operation_count_--;
	}
	return;
	}

	ReleaseLowerLevelSafepoints(T, level);
	handlers_[level]->ResetSafepointInProgress(T);
	handlers_[level]->NotifyThreadsToContinue(T);
	sl.NotifyAll();
	}
	}

	void SafepointHandler::LevelHandler::WaitUntilThreadsReachedSafepointLevel() {
	MonitorLocker sl(&parked_lock_);
	intptr_t num_attempts = 0;
	while (num_threads_not_parked_ > 0) {
	Monitor::WaitResult retval = sl.Wait(1000);
	if (retval == Monitor::kTimedOut) {
	num_attempts += 1;
	if (FLAG_trace_safepoint && num_attempts > 10) {
	for (auto current = isolate_group()->thread_registry()->active_list();
	current != nullptr; current = current->next()) {
	if (!current->IsAtSafepoint(level_)) {
	OS::PrintErr("Attempt:%" Pd " waiting for thread %s to check in\n",
	num_attempts, current->os_thread()->name());
	}
	}
	}
	}
	}
	}

	void SafepointHandler::AcquireLowerLevelSafepoints(Thread* T,
	SafepointLevel level) {
	MonitorLocker tl(threads_lock());
	ASSERT(handlers_[level]->owner_ == T);
	for (intptr_t lower_level = level - 1; lower_level >= 0; --lower_level) {
	ASSERT(!handlers_[lower_level]->SafepointInProgress());
	handlers_[lower_level]->SetSafepointInProgress(T);
	ASSERT(handlers_[lower_level]->owner_ == T);
	}
	}

	void SafepointHandler::ReleaseLowerLevelSafepoints(Thread* T,
	SafepointLevel level) {
	for (intptr_t lower_level = 0; lower_level < level; ++lower_level) {
	handlers_[lower_level]->ResetSafepointInProgress(T);
	}
	}

	void SafepointHandler::LevelHandler::NotifyThreadsToContinue(Thread* T) {
	for (auto current = isolate_group()->thread_registry()->active_list();
	current != nullptr; current = current->next()) {
	MonitorLocker tl(current->thread_lock());
	if (!current->BypassSafepoints() && current != T) {
	bool resume = false;
	for (intptr_t lower_level = level_; lower_level >= 0; --lower_level) {
	if (Thread::IsBlockedForSafepoint(current->SetSafepointRequested(
	static_cast<SafepointLevel>(lower_level), false))) {
	resume = true;
	}
	}
	if (resume) {
	tl.Notify();
	}
	}
	}
	}

	void SafepointHandler::EnterSafepointUsingLock(Thread* T) {
	MonitorLocker tl(T->thread_lock());
	EnterSafepointLocked(T, &tl, T->current_safepoint_level());
	}

	void SafepointHandler::ExitSafepointUsingLock(Thread* T) {
	MonitorLocker tl(T->thread_lock());
	ASSERT(T->IsAtSafepoint());
	ExitSafepointLocked(T, &tl, T->current_safepoint_level());
	ASSERT(!T->IsSafepointRequestedLocked(T->current_safepoint_level()));
	}

	SafepointLevel SafepointHandler::InnermostSafepointOperation(
	const Thread* current_thread) const {
	// The [current_thread] may not own the active safepoint.
	intptr_t last_count = -1;
	SafepointLevel last_level = SafepointLevel::kNoSafepoint;

	// Notice: We access SafepointLevel::{owner_,operation_count_} fields
	// without lock. This is ok since:
	//
	// * If the current thread is the owner, then it will be the one that has
	// last written `Thread::Current()` to the `owner_` field (as well as
	// updated the `operation_count_`) - nobody else can update those fields
	// in the meantime. Once the current thread exits we set it to `nullptr`.
	//
	// * If there's no owner or another thread is the owner the value cannot be
	// `Thread::Current()`: only our thread can write that particular value.
	//
	// => Even if there's racy writes by another thread, the logic is still
	// safe.
	//
	for (intptr_t level = 0; level < SafepointLevel::kNumLevels; ++level) {
	if (handlers_[level]->owner_ == current_thread) {
	const intptr_t count = handlers_[level]->operation_count_;
	if (count < last_count) return last_level;
	last_count = count;
	last_level = static_cast<SafepointLevel>(level);
	} else {
	return last_level;
	}
	}
	return last_level;
	}

	void SafepointHandler::BlockForSafepoint(Thread* T) {
	ASSERT(!T->BypassSafepoints());
	MonitorLocker tl(T->thread_lock());
	// This takes into account the safepoint level the thread can participate in.
	const SafepointLevel level = T->current_safepoint_level();
	if (T->IsSafepointRequestedLocked(level)) {
	EnterSafepointLocked(T, &tl, level);
	ExitSafepointLocked(T, &tl, level);
	ASSERT(!T->IsSafepointRequestedLocked(level));
	}
	}

	void SafepointHandler::EnterSafepointLocked(Thread* T,
	MonitorLocker* tl,
	SafepointLevel level) {
	T->SetAtSafepoint(true, level);
	// Several safepointing operations (at different) levels may happen at same
	// time. Ensure we notify all of them that we are parked now.
	for (intptr_t i = 0; i <= level; ++i) {
	if (T->IsSafepointLevelRequestedLocked(static_cast<SafepointLevel>(i))) {
	handlers_[i]->NotifyWeAreParked(T);
	}
	}
	}

	void SafepointHandler::LevelHandler::NotifyWeAreParked(Thread* T) {
	ASSERT(owner_ != nullptr);
	MonitorLocker sl(&parked_lock_);
	ASSERT(num_threads_not_parked_ > 0);
	num_threads_not_parked_ -= 1;
	if (num_threads_not_parked_ == 0) {
	sl.Notify();
	}
	}

	void SafepointHandler::ExitSafepointLocked(Thread* T,
	MonitorLocker* tl,
	SafepointLevel level) {
	while (T->IsSafepointRequestedLocked(level)) {
	T->SetBlockedForSafepoint(true);
	tl->Wait();
	T->SetBlockedForSafepoint(false);
	}
	T->SetAtSafepoint(false, level);
	}

	} // namespace dart