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

content/browser/browser_thread_impl.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 "content/browser/browser_thread_impl.h"
 
 #include <string.h>
 
 #include <string>
 
 #include "base/atomicops.h"
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/lazy_instance.h"
 #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"
 #include "content/public/browser/content_browser_client.h"
 #include "net/disk_cache/simple/simple_backend_impl.h"
 
 #if defined(OS_ANDROID)
 #include "base/android/jni_android.h"
 #endif
 
@@ skipping 64 matching lines @@
 struct BrowserThreadGlobals {
   BrowserThreadGlobals()
       : blocking_pool(new base::SequencedWorkerPool(3, "BrowserBlocking")) {
     memset(threads, 0, BrowserThread::ID_COUNT * sizeof(threads[0]));
     memset(thread_delegates, 0,
            BrowserThread::ID_COUNT * sizeof(thread_delegates[0]));
   }
 
   // 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
   // BrowserMainLoop. BrowserThreadImpl objects remove themselves from this
   // array upon destruction.
   BrowserThreadImpl* threads[BrowserThread::ID_COUNT];
 
   // Only atomic operations are used on this array. The delegates are not owned
   // by this array, rather by whoever calls BrowserThread::SetDelegate.
   BrowserThreadDelegate* thread_delegates[BrowserThread::ID_COUNT];
@@ skipping 156 matching lines @@
   BrowserThreadDelegate* delegate =
       reinterpret_cast<BrowserThreadDelegate*>(stored_pointer);
 
   if (delegate)
     delegate->CleanUp();
 
   // PostTaskHelper() accesses the message loop while holding this lock.
   // 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;
 }
 
 BrowserThreadImpl::~BrowserThreadImpl() {
   // All Thread subclasses must call Stop() in the destructor. This is
   // doubly important here as various bits of code check they are on
   // the right BrowserThread.
   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.
   for (int i = identifier_ + 1; i < ID_COUNT; ++i) {
     DCHECK(!globals.threads[i]) <<
         "Threads must be listed in the reverse order that they die";
   }
 #endif
 }
 
 bool BrowserThreadImpl::StartWithOptions(const Options& options) {
   // The global thread table needs to be locked while a new thread is
   // 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);
 }
 
 // static
 bool BrowserThreadImpl::PostTaskHelper(
     BrowserThread::ID identifier,
     const tracked_objects::Location& from_here,
     const base::Closure& task,
     base::TimeDelta delay,
     bool nestable) {
   DCHECK(identifier >= 0 && identifier < ID_COUNT);
   // Optimization: to avoid unnecessary locks, we listed the ID enumeration in
   // order of lifetime.  So no need to lock if we know that the target thread
   // outlives current thread.
   // Note: since the array is so small, ok to loop instead of creating a map,
   // which would require a lock because std::map isn't thread safe, defeating
   // the whole purpose of this optimization.
   BrowserThread::ID current_thread = ID_COUNT;
   bool target_thread_outlives_current =
       GetCurrentThreadIdentifier(&current_thread) &&
       current_thread >= identifier;
 
   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()
                                   : NULL;
   if (message_loop) {
     if (nestable) {
       message_loop->task_runner()->PostDelayedTask(from_here, task, delay);
     } else {
       message_loop->task_runner()->PostNonNestableDelayedTask(from_here, task,
                                                               delay);
     }
   }
 
   if (!target_thread_outlives_current)
-    globals.lock.Release();
+    globals.lock.ReadRelease();
 
   return !!message_loop;
 }
 
 // static
 bool BrowserThread::PostBlockingPoolTask(
     const tracked_objects::Location& from_here,
     const base::Closure& task) {
   return g_globals.Get().blocking_pool->PostWorkerTask(from_here, task);
 }
@@ skipping 29 matching lines @@
 base::SequencedWorkerPool* BrowserThread::GetBlockingPool() {
   return g_globals.Get().blocking_pool.get();
 }
 
 // static
 bool BrowserThread::IsThreadInitialized(ID identifier) {
   if (g_globals == NULL)
     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;
 }
 
 // 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() ==
              base::MessageLoop::current();
 }
 
 static const char* GetThreadName(BrowserThread::ID thread) {
   if (BrowserThread::UI < thread && thread < BrowserThread::ID_COUNT)
     return g_browser_thread_names[thread];
   if (thread == BrowserThread::UI)
@@ skipping 18 matching lines @@
   result += ".";
   return result;
 }
 
 // static
 bool BrowserThread::IsMessageLoopValid(ID identifier) {
   if (g_globals == NULL)
     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();
 }
 
 // static
 bool BrowserThread::PostTask(ID identifier,
                              const tracked_objects::Location& from_here,
                              const base::Closure& task) {
   return BrowserThreadImpl::PostTaskHelper(
@@ skipping 43 matching lines @@
 bool BrowserThread::GetCurrentThreadIdentifier(ID* identifier) {
   if (g_globals == NULL)
     return false;
 
   base::MessageLoop* cur_message_loop = base::MessageLoop::current();
   BrowserThreadGlobals& globals = g_globals.Get();
   // Profiler to track potential contention on |globals.lock|. This only does
   // 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) {
       *identifier = globals.threads[i]->identifier_;
       return true;
     }
   }
 
   return false;
 }
 
 // static
 scoped_refptr<base::SingleThreadTaskRunner>
 BrowserThread::GetMessageLoopProxyForThread(ID identifier) {
   return g_task_runners.Get().proxies[identifier];
 }
 
 // static
 base::MessageLoop* BrowserThread::UnsafeGetMessageLoopForThread(ID identifier) {
   if (g_globals == NULL)
     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();
   return loop;
 }
 
 // static
 void BrowserThread::SetDelegate(ID identifier,
                                 BrowserThreadDelegate* delegate) {
   using base::subtle::AtomicWord;
   BrowserThreadGlobals& globals = g_globals.Get();
   AtomicWord* storage = reinterpret_cast<AtomicWord*>(
       &globals.thread_delegates[identifier]);
   AtomicWord old_pointer = base::subtle::NoBarrier_AtomicExchange(
       storage, reinterpret_cast<AtomicWord>(delegate));
 
   // This catches registration when previously registered.
   DCHECK(!delegate || !old_pointer);
 }
 
 }  // namespace content