src/thread_list.cc - platform/art - Git at Google

 /*
  * Copyright (C) 2011 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 "thread_list.h"

 #include <unistd.h>

 namespace art {

 ThreadList::ThreadList(bool verbose)
     : verbose_(verbose),
       thread_list_lock_("thread list lock"),
       thread_start_cond_("thread_start_cond_"),
       thread_exit_cond_("thread_exit_cond_"),
       thread_suspend_count_lock_("thread suspend count lock"),
       thread_suspend_count_cond_("thread_suspend_count_cond_") {
 }

 ThreadList::~ThreadList() {
   // Detach the current thread if necessary.
   if (Contains(Thread::Current())) {
     Runtime::Current()->DetachCurrentThread();
   }

   WaitForNonDaemonThreadsToExit();
   SuspendAllDaemonThreads();
 }

 bool ThreadList::Contains(Thread* thread) {
   return find(list_.begin(), list_.end(), thread) != list_.end();
 }

 void ThreadList::Dump(std::ostream& os) {
   MutexLock mu(thread_list_lock_);
   os << "DALVIK THREADS (" << list_.size() << "):\n";
   for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
     (*it)->Dump(os);
     os << "\n";
   }
 }

 void ThreadList::FullSuspendCheck(Thread* thread) {
   CHECK(thread != NULL);
   CHECK_GE(thread->suspend_count_, 0);

   MutexLock mu(thread_suspend_count_lock_);
   if (thread->suspend_count_ == 0) {
     return;
   }

   if (verbose_) {
     LOG(INFO) << *thread << " self-suspending";
   }
   {
     ScopedThreadStateChange tsc(thread, Thread::kSuspended);
     while (thread->suspend_count_ != 0) {
       /*
        * Wait for wakeup signal, releasing lock.  The act of releasing
        * and re-acquiring the lock provides the memory barriers we
        * need for correct behavior on SMP.
        */
       thread_suspend_count_cond_.Wait(thread_suspend_count_lock_);
     }
     CHECK_EQ(thread->suspend_count_, 0);
   }
   if (verbose_) {
     LOG(INFO) << *thread << " self-reviving";
   }
 }

 void ThreadList::SuspendAll() {
   Thread* self = Thread::Current();

   // TODO: add another thread_suspend_lock_ to avoid GC/debugger races.

   if (verbose_) {
     LOG(INFO) << *self << " SuspendAll starting...";
   }

   MutexLock mu(thread_list_lock_);

   {
     // Increment everybody's suspend count (except our own).
     MutexLock mu(thread_suspend_count_lock_);
     for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
       Thread* thread = *it;
       if (thread != self) {
         if (verbose_) {
           LOG(INFO) << "requesting thread suspend: " << *thread;
         }
         ++thread->suspend_count_;
       }
     }
   }

   /*
    * Wait for everybody in kRunnable state to stop.  Other states
    * indicate the code is either running natively or sleeping quietly.
    * Any attempt to transition back to kRunnable will cause a check
    * for suspension, so it should be impossible for anything to execute
    * interpreted code or modify objects (assuming native code plays nicely).
    *
    * It's also okay if the thread transitions to a non-kRunnable state.
    *
    * Note we released the thread_suspend_count_lock_ before getting here,
    * so if another thread is fiddling with its suspend count (perhaps
    * self-suspending for the debugger) it won't block while we're waiting
    * in here.
    */
   for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
     Thread* thread = *it;
     if (thread != self) {
       thread->WaitUntilSuspended();
       if (verbose_) {
         LOG(INFO) << "thread suspended: " << *thread;
       }
     }
   }

   if (verbose_) {
     LOG(INFO) << *self << " SuspendAll complete";
   }
 }

 void ThreadList::Suspend(Thread* thread) {
   DCHECK(thread != Thread::Current());

   // TODO: add another thread_suspend_lock_ to avoid GC/debugger races.

   if (verbose_) {
     LOG(INFO) << "Suspend(" << *thread << ") starting...";
   }

   MutexLock mu(thread_list_lock_);
   if (!Contains(thread)) {
     return;
   }

   {
     MutexLock mu(thread_suspend_count_lock_);
     ++thread->suspend_count_;
   }

   thread->WaitUntilSuspended();

   if (verbose_) {
     LOG(INFO) << "Suspend(" << *thread << ") complete";
   }
 }


 void ThreadList::ResumeAll() {
   Thread* self = Thread::Current();

   if (verbose_) {
     LOG(INFO) << *self << " ResumeAll starting";
   }

   // Decrement the suspend counts for all threads.  No need for atomic
   // writes, since nobody should be moving until we decrement the count.
   // We do need to hold the thread list because of JNI attaches.
   {
     MutexLock mu1(thread_list_lock_);
     MutexLock mu2(thread_suspend_count_lock_);
     for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
       Thread* thread = *it;
       if (thread != self) {
         if (thread->suspend_count_ > 0) {
           --thread->suspend_count_;
         } else {
           LOG(WARNING) << *thread << " suspend count already zero";
         }
       }
     }
   }

   // Broadcast a notification to all suspended threads, some or all of
   // which may choose to wake up.  No need to wait for them.
   {
     if (verbose_) {
       LOG(INFO) << *self << " ResumeAll waking others";
     }
     MutexLock mu(thread_suspend_count_lock_);
     thread_suspend_count_cond_.Broadcast();
   }

   if (verbose_) {
     LOG(INFO) << *self << " ResumeAll complete";
   }
 }

 void ThreadList::Resume(Thread* thread) {
   DCHECK(thread != Thread::Current());

   if (verbose_) {
     LOG(INFO) << "Resume(" << *thread << ") starting...";
   }

   {
     MutexLock mu1(thread_list_lock_);
     MutexLock mu2(thread_suspend_count_lock_);
     if (!Contains(thread)) {
       return;
     }
     if (thread->suspend_count_ > 0) {
       --thread->suspend_count_;
     } else {
       LOG(WARNING) << *thread << " suspend count already zero";
     }
   }

   {
     if (verbose_) {
       LOG(INFO) << "Resume(" << *thread << ") waking others";
     }
     MutexLock mu(thread_suspend_count_lock_);
     thread_suspend_count_cond_.Broadcast();
   }

   if (verbose_) {
     LOG(INFO) << "Resume(" << *thread << ") complete";
   }
 }

 void ThreadList::RunWhileSuspended(Thread* thread, void (*callback)(void*), void* arg) {
   DCHECK(thread != NULL);
   Thread* self = Thread::Current();
   if (thread != self) {
     Suspend(thread);
   }
   callback(arg);
   if (thread != self) {
     Resume(thread);
   }
 }

 void ThreadList::Register() {
   Thread* self = Thread::Current();

   if (verbose_) {
     LOG(INFO) << "ThreadList::Register() " << *self;
     self->Dump(std::cerr);
   }

   MutexLock mu(thread_list_lock_);
   CHECK(!Contains(self));
   list_.push_back(self);
 }

 void ThreadList::Unregister() {
   Thread* self = Thread::Current();

   if (verbose_) {
     LOG(INFO) << "ThreadList::Unregister() " << *self;
   }

   MutexLock mu(thread_list_lock_);

   // Remove this thread from the list.
   CHECK(Contains(self));
   list_.remove(self);

   // Delete the Thread* and release the thin lock id.
   uint32_t thin_lock_id = self->thin_lock_id_;
   delete self;
   ReleaseThreadId(thin_lock_id);

   // Clear the TLS data, so that thread is recognizably detached.
   // (It may wish to reattach later.)
   CHECK_PTHREAD_CALL(pthread_setspecific, (Thread::pthread_key_self_, NULL), "detach self");

   // Signal that a thread just detached.
   thread_exit_cond_.Signal();
 }

 void ThreadList::VisitRoots(Heap::RootVisitor* visitor, void* arg) const {
   MutexLock mu(thread_list_lock_);
   for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
     (*it)->VisitRoots(visitor, arg);
   }
 }

 /*
  * Tell a new thread it's safe to start.
  *
  * We must hold the thread list lock before messing with another thread.
  * In the general case we would also need to verify that the new thread was
  * still in the thread list, but in our case the thread has not started
  * executing user code and therefore has not had a chance to exit.
  *
  * We move it to kVmWait, and it then shifts itself to kRunning, which
  * comes with a suspend-pending check. We do this after
  */
 void ThreadList::SignalGo(Thread* child) {
   Thread* self = Thread::Current();
   CHECK(child != self);

   {
     MutexLock mu(thread_list_lock_);

     // We wait for the child to tell us that it's in the thread list.
     while (child->GetState() != Thread::kStarting) {
       thread_start_cond_.Wait(thread_list_lock_);
     }
   }

   // If we switch out of runnable and then back in, we know there's no pending suspend.
   self->SetState(Thread::kVmWait);
   self->SetState(Thread::kRunnable);

   // Tell the child that it's safe: it will see any future suspend request.
   child->SetState(Thread::kVmWait);
   thread_start_cond_.Broadcast();
 }

 void ThreadList::WaitForGo() {
   Thread* self = Thread::Current();
   DCHECK(Contains(self));

   MutexLock mu(thread_list_lock_);

   // Tell our parent that we're in the thread list.
   self->SetState(Thread::kStarting);
   thread_start_cond_.Broadcast();

   // Wait until our parent tells us there's no suspend still pending
   // from before we were on the thread list.
   while (self->GetState() != Thread::kVmWait) {
     thread_start_cond_.Wait(thread_list_lock_);
   }

   // Enter the runnable state. We know that any pending suspend will affect us now.
   self->SetState(Thread::kRunnable);
 }

 bool ThreadList::AllThreadsAreDaemons() {
   for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
     // TODO: there's a race here with thread exit that's being worked around by checking if the peer
     // is null.
     if ((*it)->GetPeer() != NULL && !(*it)->IsDaemon()) {
       return false;
     }
   }
   return true;
 }

 void ThreadList::WaitForNonDaemonThreadsToExit() {
   MutexLock mu(thread_list_lock_);
   while (!AllThreadsAreDaemons()) {
     thread_exit_cond_.Wait(thread_list_lock_);
   }
 }

 void ThreadList::SuspendAllDaemonThreads() {
   MutexLock mu(thread_list_lock_);

   // Tell all the daemons it's time to suspend. (At this point, we know
   // all threads are daemons.)
   {
     MutexLock mu(thread_suspend_count_lock_);
     for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
       Thread* thread = *it;
       ++thread->suspend_count_;
     }
   }

   // Give the threads a chance to suspend, complaining if they're slow.
   bool have_complained = false;
   for (int i = 0; i < 10; ++i) {
     usleep(200 * 1000);
     bool all_suspended = true;
     for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
       Thread* thread = *it;
       if (thread->GetState() == Thread::kRunnable) {
         if (!have_complained) {
           LOG(WARNING) << "daemon thread not yet suspended: " << *thread;
           have_complained = true;
         }
         all_suspended = false;
       }
     }
     if (all_suspended) {
       return;
     }
   }
 }

 uint32_t ThreadList::AllocThreadId() {
   MutexLock mu(thread_list_lock_);
   for (size_t i = 0; i < allocated_ids_.size(); ++i) {
     if (!allocated_ids_[i]) {
       allocated_ids_.set(i);
       return i + 1; // Zero is reserved to mean "invalid".
     }
   }
   LOG(FATAL) << "Out of internal thread ids";
   return 0;
 }

 void ThreadList::ReleaseThreadId(uint32_t id) {
   thread_list_lock_.AssertHeld();
   --id; // Zero is reserved to mean "invalid".
   DCHECK(allocated_ids_[id]) << id;
   allocated_ids_.reset(id);
 }

 }  // namespace art
	/*
	* Copyright (C) 2011 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 "thread_list.h"

	#include <unistd.h>

	namespace art {

	ThreadList::ThreadList(bool verbose)
	: verbose_(verbose),
	thread_list_lock_("thread list lock"),
	thread_start_cond_("thread_start_cond_"),
	thread_exit_cond_("thread_exit_cond_"),
	thread_suspend_count_lock_("thread suspend count lock"),
	thread_suspend_count_cond_("thread_suspend_count_cond_") {
	}

	ThreadList::~ThreadList() {
	// Detach the current thread if necessary.
	if (Contains(Thread::Current())) {
	Runtime::Current()->DetachCurrentThread();
	}

	WaitForNonDaemonThreadsToExit();
	SuspendAllDaemonThreads();
	}

	bool ThreadList::Contains(Thread* thread) {
	return find(list_.begin(), list_.end(), thread) != list_.end();
	}

	void ThreadList::Dump(std::ostream& os) {
	MutexLock mu(thread_list_lock_);
	os << "DALVIK THREADS (" << list_.size() << "):\n";
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	(*it)->Dump(os);
	os << "\n";
	}
	}

	void ThreadList::FullSuspendCheck(Thread* thread) {
	CHECK(thread != NULL);
	CHECK_GE(thread->suspend_count_, 0);

	MutexLock mu(thread_suspend_count_lock_);
	if (thread->suspend_count_ == 0) {
	return;
	}

	if (verbose_) {
	LOG(INFO) << *thread << " self-suspending";
	}
	{
	ScopedThreadStateChange tsc(thread, Thread::kSuspended);
	while (thread->suspend_count_ != 0) {
	/*
	* Wait for wakeup signal, releasing lock. The act of releasing
	* and re-acquiring the lock provides the memory barriers we
	* need for correct behavior on SMP.
	*/
	thread_suspend_count_cond_.Wait(thread_suspend_count_lock_);
	}
	CHECK_EQ(thread->suspend_count_, 0);
	}
	if (verbose_) {
	LOG(INFO) << *thread << " self-reviving";
	}
	}

	void ThreadList::SuspendAll() {
	Thread* self = Thread::Current();

	// TODO: add another thread_suspend_lock_ to avoid GC/debugger races.

	if (verbose_) {
	LOG(INFO) << *self << " SuspendAll starting...";
	}

	MutexLock mu(thread_list_lock_);

	{
	// Increment everybody's suspend count (except our own).
	MutexLock mu(thread_suspend_count_lock_);
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	Thread* thread = *it;
	if (thread != self) {
	if (verbose_) {
	LOG(INFO) << "requesting thread suspend: " << *thread;
	}
	++thread->suspend_count_;
	}
	}
	}

	/*
	* Wait for everybody in kRunnable state to stop. Other states
	* indicate the code is either running natively or sleeping quietly.
	* Any attempt to transition back to kRunnable will cause a check
	* for suspension, so it should be impossible for anything to execute
	* interpreted code or modify objects (assuming native code plays nicely).
	*
	* It's also okay if the thread transitions to a non-kRunnable state.
	*
	* Note we released the thread_suspend_count_lock_ before getting here,
	* so if another thread is fiddling with its suspend count (perhaps
	* self-suspending for the debugger) it won't block while we're waiting
	* in here.
	*/
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	Thread* thread = *it;
	if (thread != self) {
	thread->WaitUntilSuspended();
	if (verbose_) {
	LOG(INFO) << "thread suspended: " << *thread;
	}
	}
	}

	if (verbose_) {
	LOG(INFO) << *self << " SuspendAll complete";
	}
	}

	void ThreadList::Suspend(Thread* thread) {
	DCHECK(thread != Thread::Current());

	// TODO: add another thread_suspend_lock_ to avoid GC/debugger races.

	if (verbose_) {
	LOG(INFO) << "Suspend(" << *thread << ") starting...";
	}

	MutexLock mu(thread_list_lock_);
	if (!Contains(thread)) {
	return;
	}

	{
	MutexLock mu(thread_suspend_count_lock_);
	++thread->suspend_count_;
	}

	thread->WaitUntilSuspended();

	if (verbose_) {
	LOG(INFO) << "Suspend(" << *thread << ") complete";
	}
	}


	void ThreadList::ResumeAll() {
	Thread* self = Thread::Current();

	if (verbose_) {
	LOG(INFO) << *self << " ResumeAll starting";
	}

	// Decrement the suspend counts for all threads. No need for atomic
	// writes, since nobody should be moving until we decrement the count.
	// We do need to hold the thread list because of JNI attaches.
	{
	MutexLock mu1(thread_list_lock_);
	MutexLock mu2(thread_suspend_count_lock_);
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	Thread* thread = *it;
	if (thread != self) {
	if (thread->suspend_count_ > 0) {
	--thread->suspend_count_;
	} else {
	LOG(WARNING) << *thread << " suspend count already zero";
	}
	}
	}
	}

	// Broadcast a notification to all suspended threads, some or all of
	// which may choose to wake up. No need to wait for them.
	{
	if (verbose_) {
	LOG(INFO) << *self << " ResumeAll waking others";
	}
	MutexLock mu(thread_suspend_count_lock_);
	thread_suspend_count_cond_.Broadcast();
	}

	if (verbose_) {
	LOG(INFO) << *self << " ResumeAll complete";
	}
	}

	void ThreadList::Resume(Thread* thread) {
	DCHECK(thread != Thread::Current());

	if (verbose_) {
	LOG(INFO) << "Resume(" << *thread << ") starting...";
	}

	{
	MutexLock mu1(thread_list_lock_);
	MutexLock mu2(thread_suspend_count_lock_);
	if (!Contains(thread)) {
	return;
	}
	if (thread->suspend_count_ > 0) {
	--thread->suspend_count_;
	} else {
	LOG(WARNING) << *thread << " suspend count already zero";
	}
	}

	{
	if (verbose_) {
	LOG(INFO) << "Resume(" << *thread << ") waking others";
	}
	MutexLock mu(thread_suspend_count_lock_);
	thread_suspend_count_cond_.Broadcast();
	}

	if (verbose_) {
	LOG(INFO) << "Resume(" << *thread << ") complete";
	}
	}

	void ThreadList::RunWhileSuspended(Thread* thread, void (callback)(void), void* arg) {
	DCHECK(thread != NULL);
	Thread* self = Thread::Current();
	if (thread != self) {
	Suspend(thread);
	}
	callback(arg);
	if (thread != self) {
	Resume(thread);
	}
	}

	void ThreadList::Register() {
	Thread* self = Thread::Current();

	if (verbose_) {
	LOG(INFO) << "ThreadList::Register() " << *self;
	self->Dump(std::cerr);
	}

	MutexLock mu(thread_list_lock_);
	CHECK(!Contains(self));
	list_.push_back(self);
	}

	void ThreadList::Unregister() {
	Thread* self = Thread::Current();

	if (verbose_) {
	LOG(INFO) << "ThreadList::Unregister() " << *self;
	}

	MutexLock mu(thread_list_lock_);

	// Remove this thread from the list.
	CHECK(Contains(self));
	list_.remove(self);

	// Delete the Thread* and release the thin lock id.
	uint32_t thin_lock_id = self->thin_lock_id_;
	delete self;
	ReleaseThreadId(thin_lock_id);

	// Clear the TLS data, so that thread is recognizably detached.
	// (It may wish to reattach later.)
	CHECK_PTHREAD_CALL(pthread_setspecific, (Thread::pthread_key_self_, NULL), "detach self");

	// Signal that a thread just detached.
	thread_exit_cond_.Signal();
	}

	void ThreadList::VisitRoots(Heap::RootVisitor* visitor, void* arg) const {
	MutexLock mu(thread_list_lock_);
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	(*it)->VisitRoots(visitor, arg);
	}
	}

	/*
	* Tell a new thread it's safe to start.
	*
	* We must hold the thread list lock before messing with another thread.
	* In the general case we would also need to verify that the new thread was
	* still in the thread list, but in our case the thread has not started
	* executing user code and therefore has not had a chance to exit.
	*
	* We move it to kVmWait, and it then shifts itself to kRunning, which
	* comes with a suspend-pending check. We do this after
	*/
	void ThreadList::SignalGo(Thread* child) {
	Thread* self = Thread::Current();
	CHECK(child != self);

	{
	MutexLock mu(thread_list_lock_);

	// We wait for the child to tell us that it's in the thread list.
	while (child->GetState() != Thread::kStarting) {
	thread_start_cond_.Wait(thread_list_lock_);
	}
	}

	// If we switch out of runnable and then back in, we know there's no pending suspend.
	self->SetState(Thread::kVmWait);
	self->SetState(Thread::kRunnable);

	// Tell the child that it's safe: it will see any future suspend request.
	child->SetState(Thread::kVmWait);
	thread_start_cond_.Broadcast();
	}

	void ThreadList::WaitForGo() {
	Thread* self = Thread::Current();
	DCHECK(Contains(self));

	MutexLock mu(thread_list_lock_);

	// Tell our parent that we're in the thread list.
	self->SetState(Thread::kStarting);
	thread_start_cond_.Broadcast();

	// Wait until our parent tells us there's no suspend still pending
	// from before we were on the thread list.
	while (self->GetState() != Thread::kVmWait) {
	thread_start_cond_.Wait(thread_list_lock_);
	}

	// Enter the runnable state. We know that any pending suspend will affect us now.
	self->SetState(Thread::kRunnable);
	}

	bool ThreadList::AllThreadsAreDaemons() {
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	// TODO: there's a race here with thread exit that's being worked around by checking if the peer
	// is null.
	if ((it)->GetPeer() != NULL && !(it)->IsDaemon()) {
	return false;
	}
	}
	return true;
	}

	void ThreadList::WaitForNonDaemonThreadsToExit() {
	MutexLock mu(thread_list_lock_);
	while (!AllThreadsAreDaemons()) {
	thread_exit_cond_.Wait(thread_list_lock_);
	}
	}

	void ThreadList::SuspendAllDaemonThreads() {
	MutexLock mu(thread_list_lock_);

	// Tell all the daemons it's time to suspend. (At this point, we know
	// all threads are daemons.)
	{
	MutexLock mu(thread_suspend_count_lock_);
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	Thread* thread = *it;
	++thread->suspend_count_;
	}
	}

	// Give the threads a chance to suspend, complaining if they're slow.
	bool have_complained = false;
	for (int i = 0; i < 10; ++i) {
	usleep(200 * 1000);
	bool all_suspended = true;
	for (It it = list_.begin(), end = list_.end(); it != end; ++it) {
	Thread* thread = *it;
	if (thread->GetState() == Thread::kRunnable) {
	if (!have_complained) {
	LOG(WARNING) << "daemon thread not yet suspended: " << *thread;
	have_complained = true;
	}
	all_suspended = false;
	}
	}
	if (all_suspended) {
	return;
	}
	}
	}

	uint32_t ThreadList::AllocThreadId() {
	MutexLock mu(thread_list_lock_);
	for (size_t i = 0; i < allocated_ids_.size(); ++i) {
	if (!allocated_ids_[i]) {
	allocated_ids_.set(i);
	return i + 1; // Zero is reserved to mean "invalid".
	}
	}
	LOG(FATAL) << "Out of internal thread ids";
	return 0;
	}

	void ThreadList::ReleaseThreadId(uint32_t id) {
	thread_list_lock_.AssertHeld();
	--id; // Zero is reserved to mean "invalid".
	DCHECK(allocated_ids_[id]) << id;
	allocated_ids_.reset(id);
	}

	} // namespace art