Use a reader-writer lock to protect the threads table in BrowserThreadImpl.

content/browser/browser_thread_impl.cc

Index: content/browser/browser_thread_impl.cc
diff --git a/content/browser/browser_thread_impl.cc b/content/browser/browser_thread_impl.cc
index 90b581882fb7bfb0fc35c332215e1a677b364f6b..6ff498e96d41a6a1f171fb91d807407a2e05e14a 100644
--- a/content/browser/browser_thread_impl.cc
+++ b/content/browser/browser_thread_impl.cc
@@ -15,6 +15,7 @@
 #include "base/macros.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/single_thread_task_runner.h"
+#include "base/synchronization/read_write_lock.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "build/build_config.h"
 #include "content/public/browser/browser_thread_delegate.h"
@@ -99,7 +100,7 @@ struct BrowserThreadGlobals {
 
   // This lock protects |threads|. Do not read or modify that array
   // without holding this lock. Do not block while holding this lock.
-  base::Lock lock;
+  base::ReadWriteLock lock;
 
   // This array is protected by |lock|. The threads are not owned by this
   // array. Typically, the threads are owned on the UI thread by
@@ -276,14 +277,14 @@ void BrowserThreadImpl::CleanUp() {
   // However, the message loop will soon be destructed without any locking. So
   // to prevent a race with accessing the message loop in PostTaskHelper(),
   // remove this thread from the global array now.
-  base::AutoLock lock(globals.lock);
+  base::AutoWriteLock lock(globals.lock);
   globals.threads[identifier_] = NULL;
 }
 
 void BrowserThreadImpl::Initialize() {
   BrowserThreadGlobals& globals = g_globals.Get();
 
-  base::AutoLock lock(globals.lock);
+  base::AutoWriteLock lock(globals.lock);
   DCHECK(identifier_ >= 0 && identifier_ < ID_COUNT);
   DCHECK(globals.threads[identifier_] == NULL);
   globals.threads[identifier_] = this;
@@ -296,7 +297,7 @@ BrowserThreadImpl::~BrowserThreadImpl() {
   Stop();
 
   BrowserThreadGlobals& globals = g_globals.Get();
-  base::AutoLock lock(globals.lock);
+  base::AutoWriteLock lock(globals.lock);
   globals.threads[identifier_] = NULL;
 #ifndef NDEBUG
   // Double check that the threads are ordered correctly in the enumeration.
@@ -312,7 +313,7 @@ bool BrowserThreadImpl::StartWithOptions(const Options& options) {
   // starting, as the new thread can asynchronously start touching the
   // table (and other thread's message_loop).
   BrowserThreadGlobals& globals = g_globals.Get();
-  base::AutoLock lock(globals.lock);
+  base::AutoWriteLock lock(globals.lock);
   return Thread::StartWithOptions(options);
 }
 
@@ -337,7 +338,7 @@ bool BrowserThreadImpl::PostTaskHelper(
 
   BrowserThreadGlobals& globals = g_globals.Get();
   if (!target_thread_outlives_current)
-    globals.lock.Acquire();
+    globals.lock.ReadAcquire();
 
   base::MessageLoop* message_loop =
       globals.threads[identifier] ? globals.threads[identifier]->message_loop()
@@ -352,7 +353,7 @@ bool BrowserThreadImpl::PostTaskHelper(
   }
 
   if (!target_thread_outlives_current)
-    globals.lock.Release();
+    globals.lock.ReadRelease();
 
   return !!message_loop;
 }
@@ -402,7 +403,7 @@ bool BrowserThread::IsThreadInitialized(ID identifier) {
     return false;
 
   BrowserThreadGlobals& globals = g_globals.Get();
-  base::AutoLock lock(globals.lock);
+  base::AutoReadLock lock(globals.lock);
   DCHECK(identifier >= 0 && identifier < ID_COUNT);
   return globals.threads[identifier] != NULL;
 }
@@ -410,7 +411,7 @@ bool BrowserThread::IsThreadInitialized(ID identifier) {
 // static
 bool BrowserThread::CurrentlyOn(ID identifier) {
   BrowserThreadGlobals& globals = g_globals.Get();
-  base::AutoLock lock(globals.lock);
+  base::AutoReadLock lock(globals.lock);
   DCHECK(identifier >= 0 && identifier < ID_COUNT);
   return globals.threads[identifier] &&
          globals.threads[identifier]->message_loop() ==
@@ -449,7 +450,7 @@ bool BrowserThread::IsMessageLoopValid(ID identifier) {
     return false;
 
   BrowserThreadGlobals& globals = g_globals.Get();
-  base::AutoLock lock(globals.lock);
+  base::AutoReadLock lock(globals.lock);
   DCHECK(identifier >= 0 && identifier < ID_COUNT);
   return globals.threads[identifier] &&
          globals.threads[identifier]->message_loop();
@@ -513,7 +514,7 @@ bool BrowserThread::GetCurrentThreadIdentifier(ID* identifier) {
   // real work on canary and local dev builds, so the cost of having this here
   // should be minimal.
   tracked_objects::ScopedTracker tracking_profile(FROM_HERE);
-  base::AutoLock lock(globals.lock);
+  base::AutoReadLock lock(globals.lock);
   for (int i = 0; i < ID_COUNT; ++i) {
     if (globals.threads[i] &&
         globals.threads[i]->message_loop() == cur_message_loop) {
@@ -537,7 +538,7 @@ base::MessageLoop* BrowserThread::UnsafeGetMessageLoopForThread(ID identifier) {
     return NULL;
 
   BrowserThreadGlobals& globals = g_globals.Get();
-  base::AutoLock lock(globals.lock);
+  base::AutoReadLock lock(globals.lock);
   base::Thread* thread = globals.threads[identifier];
   DCHECK(thread);
   base::MessageLoop* loop = thread->message_loop();