PlatformThreadHandle: remove public id() interface

base/threading/thread.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/threading/thread.h"
 
 #include "base/bind.h"
 #include "base/lazy_instance.h"
 #include "base/location.h"
 #include "base/synchronization/waitable_event.h"
@@ skipping 44 matching lines @@
 }
 
 Thread::Thread(const std::string& name)
     :
 #if defined(OS_WIN)
       com_status_(NONE),
 #endif
       stopping_(false),
       running_(false),
       thread_(0),
+      id_(kInvalidThreadId),
       message_loop_(nullptr),
       message_loop_timer_slack_(TIMER_SLACK_NONE),
       name_(name) {
 }
 
 Thread::~Thread() {
   Stop();
 }
 
 bool Thread::Start() {
@@ skipping 15 matching lines @@
   SetThreadWasQuitProperly(false);
 
   MessageLoop::Type type = options.message_loop_type;
   if (!options.message_pump_factory.is_null())
     type = MessageLoop::TYPE_CUSTOM;
 
   message_loop_timer_slack_ = options.timer_slack;
   scoped_ptr<MessageLoop> message_loop = MessageLoop::CreateUnbound(
       type, options.message_pump_factory);
   message_loop_ = message_loop.get();
-  start_event_.reset(new WaitableEvent(false, false));
+  id_ = kInvalidThreadId;
+  start_event_.reset(new WaitableEvent(true, false));
 
-  // Hold the thread_lock_ while starting a new thread, so that we can make sure
-  // that thread_ is populated before the newly created thread accesses it.
-  {
-    AutoLock lock(thread_lock_);
-    bool created;
-    if (options.priority == ThreadPriority::NORMAL) {
-      created = PlatformThread::Create(options.stack_size, this, &thread_);
-    } else {
-      created = PlatformThread::CreateWithPriority(options.stack_size, this,
-                                                   &thread_, options.priority);
-    }
-    if (!created) {
-      DLOG(ERROR) << "failed to create thread";
-      message_loop_ = nullptr;
-      start_event_.reset();
-      return false;
-    }
+  if (!PlatformThread::CreateWithPriority(options.stack_size, this, &thread_,
+                                          options.priority)) {
+    DLOG(ERROR) << "failed to create thread";
+    message_loop_ = nullptr;
+    start_event_.reset();
+    return false;
   }
 
   // The ownership of message_loop is managemed by the newly created thread
   // within the ThreadMain.
   ignore_result(message_loop.release());
 
   DCHECK(message_loop_);
   return true;
 }
 
 bool Thread::StartAndWaitForTesting() {
   bool result = Start();
   if (!result)
     return false;
   WaitUntilThreadStarted();
   return true;
 }
 
-bool Thread::WaitUntilThreadStarted() {
+bool Thread::WaitUntilThreadStarted() const {
   if (!start_event_)
     return false;
   base::ThreadRestrictions::ScopedAllowWait allow_wait;
   start_event_->Wait();
   return true;
 }
 
 void Thread::Stop() {
   if (!start_event_)
     return;
@@ skipping 10 matching lines @@
   // The thread should NULL message_loop_ on exit.
   DCHECK(!message_loop_);
 
   // The thread no longer needs to be joined.
   start_event_.reset();
 
   stopping_ = false;
 }
 
 void Thread::StopSoon() {
-  // We should only be called on the same thread that started us.
-
-  DCHECK_NE(thread_id(), PlatformThread::CurrentId());
-
   if (stopping_ || !message_loop_)
     return;
 
+  // We should only be called on the same thread that started us.
+  DCHECK_NE(thread_id(), PlatformThread::CurrentId());

[gab, 2015/07/16 17:19:33]

Why move this? DCHECKs enforcing the method's contract typically go at the top.

[Takashi Toyoshima, 2015/07/16 19:51:11]

Because without this fix, this assertion fires in a test that calls Start() and
Stop() repeatedly. But I have another right fix in the newer patch set, and this
is already reverted.

+
   stopping_ = true;
   task_runner()->PostTask(FROM_HERE, base::Bind(&ThreadQuitHelper));
 }
 
 PlatformThreadId Thread::thread_id() const {
-  AutoLock lock(thread_lock_);
-  return thread_.id();
+  WaitUntilThreadStarted();
+  // Could fire if the method is called before starting the thread.
+  DCHECK_NE(kInvalidThreadId, id_);
+  return id_;
 }
 
 bool Thread::IsRunning() const {
   // If the thread's already started (i.e. message_loop_ is non-null) and
   // not yet requested to stop (i.e. stopping_ is false) we can just return
   // true. (Note that stopping_ is touched only on the same thread that
   // starts / started the new thread so we need no locking here.)
   if (message_loop_ && !stopping_)
     return true;
   // Otherwise check the running_ flag, which is set to true by the new thread
@@ skipping 32 matching lines @@
 
 #if defined(OS_WIN)
   scoped_ptr<win::ScopedCOMInitializer> com_initializer;
   if (com_status_ != NONE) {
     com_initializer.reset((com_status_ == STA) ?
         new win::ScopedCOMInitializer() :
         new win::ScopedCOMInitializer(win::ScopedCOMInitializer::kMTA));
   }
 #endif
 
-  // Make sure the thread_id() returns current thread.
-  // (This internally acquires lock against PlatformThread::Create)
-  DCHECK_EQ(thread_id(), PlatformThread::CurrentId());
-
   // Let the thread do extra initialization.
   Init();
 
   {
     AutoLock lock(running_lock_);
     running_ = true;
   }
 
+  id_ = PlatformThread::CurrentId();

[gab, 2015/07/16 17:19:33]

You still need a lock to protect this even if all the reads are after the only
write.

Well, you don't actually need a "lock", you need a "memory barrier" to force
other threads to see the new value (even if they never read it explicitly they
could have cached a page that contained it) -- and a "lock" is the simplest
version of such a barrier in our codebase (plus in non-contentious scenarios,
which you are creating here, there is pretty much zero overhead to a lock).

[Takashi Toyoshima, 2015/07/16 19:51:10]

Yeah, I understand memory consistency. As I commented in the header,
start_event_->Signal() implicitly performs membar operation. So this should
work. But the problem here is the original code does not protect start_event_.
scoped_ptr<WaitableEvent> must need a lock to access the internal pointer, but
we set and reset the pointer on multiple threads without any lock.

The reason why I use a event instead of lock is simple. id_ could not be
available until the created thread actually starts running. So reading id_ needs
to wait for the start_event_ signaled anyway.

+  DCHECK_NE(kInvalidThreadId, id_);
+
   start_event_->Signal();
 
   Run(message_loop_);
 
   {
     AutoLock lock(running_lock_);
     running_ = false;
   }
 
   // Let the thread do extra cleanup.
   CleanUp();
 
 #if defined(OS_WIN)
   com_initializer.reset();
 #endif
 
   // Assert that MessageLoop::Quit was called by ThreadQuitHelper.
   DCHECK(GetThreadWasQuitProperly());
 
   // We can't receive messages anymore.
   // (The message loop is destructed at the end of this block)
-  message_loop_ = NULL;
+  message_loop_ = nullptr;
 }
 
 }  // namespace base