Make Profiler::tail_ atomic

it's used on several threads

BUG=none
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/639763002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24465 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/log.cc

@@ -607,7 +607,7 @@ class Profiler: public base::Thread {
     if (paused_)
       return;
 
-    if (Succ(head_) == tail_) {
+    if (Succ(head_) == static_cast<int>(base::NoBarrier_Load(&tail_))) {
       overflow_ = true;
     } else {
       buffer_[head_] = *sample;
@@ -626,9 +626,10 @@ class Profiler: public base::Thread {
   // Waits for a signal and removes profiling data.
   bool Remove(TickSample* sample) {
     buffer_semaphore_.Wait();  // Wait for an element.
-    *sample = buffer_[tail_];
+    *sample = buffer_[base::NoBarrier_Load(&tail_)];
     bool result = overflow_;
-    tail_ = Succ(tail_);
+    base::NoBarrier_Store(&tail_, static_cast<base::Atomic32>(
+                                      Succ(base::NoBarrier_Load(&tail_))));
     overflow_ = false;
     return result;
   }
@@ -642,7 +643,7 @@ class Profiler: public base::Thread {
   static const int kBufferSize = 128;
   TickSample buffer_[kBufferSize];  // Buffer storage.
   int head_;  // Index to the buffer head.
-  int tail_;  // Index to the buffer tail.
+  base::Atomic32 tail_;             // Index to the buffer tail.
   bool overflow_;  // Tell whether a buffer overflow has occurred.
   // Sempahore used for buffer synchronization.
   base::Semaphore buffer_semaphore_;
@@ -699,12 +700,13 @@ Profiler::Profiler(Isolate* isolate)
     : base::Thread(Options("v8:Profiler")),
       isolate_(isolate),
       head_(0),
-      tail_(0),
       overflow_(false),
       buffer_semaphore_(0),
       engaged_(false),
       running_(false),
-      paused_(false) {}
+      paused_(false) {
+  base::NoBarrier_Store(&tail_, 0);
+}
 
 
 void Profiler::Engage() {