Reduce the scope of rtc::Event::Wait() locking.

webrtc/base/event.cc

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 61 matching lines @@
   pthread_mutex_unlock(&event_mutex_);
 }
 
 void Event::Reset() {
   pthread_mutex_lock(&event_mutex_);
   event_status_ = false;
   pthread_mutex_unlock(&event_mutex_);
 }
 
 bool Event::Wait(int milliseconds) {
-  pthread_mutex_lock(&event_mutex_);
   int error = 0;
 
+  struct timespec ts;
   if (milliseconds != kForever) {
     // Converting from seconds and microseconds (1e-6) plus
     // milliseconds (1e-3) to seconds and nanoseconds (1e-9).
 
-    struct timespec ts;
 #if HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
     // Use relative time version, which tends to be more efficient for
     // pthread implementations where provided (like on Android).
     ts.tv_sec = milliseconds / 1000;
     ts.tv_nsec = (milliseconds % 1000) * 1000000;
 #else
     struct timeval tv;
     gettimeofday(&tv, NULL);
 
     ts.tv_sec = tv.tv_sec + (milliseconds / 1000);
     ts.tv_nsec = tv.tv_usec * 1000 + (milliseconds % 1000) * 1000000;
 
     // Handle overflow.
     if (ts.tv_nsec >= 1000000000) {
       ts.tv_sec++;
       ts.tv_nsec -= 1000000000;
     }
 #endif
+  }
 
+  pthread_mutex_lock(&event_mutex_);

[tommi, 2016/02/19 16:40:42]

There's a slight change in behavior because of this.
With the patch the timeout *includes* the time it takes to wait for
event_mutex_, but without it, the cost of acquiring event_mutex_ is not counted
as part of doing the actual wait.
Maybe that does matter, maybe not, but we need to keep an eye on it.

+  if (milliseconds != kForever) {
     while (!event_status_ && error == 0) {
 #if HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
       error = pthread_cond_timedwait_relative_np(
           &event_cond_, &event_mutex_, &ts);
 #else
       error = pthread_cond_timedwait(&event_cond_, &event_mutex_, &ts);
 #endif
     }
   } else {
     while (!event_status_ && error == 0)
       error = pthread_cond_wait(&event_cond_, &event_mutex_);
   }
 
   // NOTE(liulk): Exactly one thread will auto-reset this event. All
   // the other threads will think it's unsignaled.  This seems to be
   // consistent with auto-reset events in WEBRTC_WIN
   if (error == 0 && !is_manual_reset_)
     event_status_ = false;
 
   pthread_mutex_unlock(&event_mutex_);
 
   return (error == 0);
 }
 
 #endif
 
 }  // namespace rtc