Implement store as an explicit atomic operation.

webrtc/base/atomicops.h

Index: webrtc/base/atomicops.h
diff --git a/webrtc/base/atomicops.h b/webrtc/base/atomicops.h
index 80b212444305143229b0de5bc54f4bbaa0c7ddeb..416f6e66fe46047d078f7be2005e496facc5ad42 100644
--- a/webrtc/base/atomicops.h
+++ b/webrtc/base/atomicops.h
@@ -19,6 +19,10 @@
 #include <winsock2.h>
 #include <windows.h>
 #endif  // defined(WEBRTC_WIN)
+#if defined(THREAD_SANITIZER)
+#include <sanitizer/tsan_interface_atomic.h>
+#include <stdint.h>
+#endif  // defined(THREAD_SANITIZER)
 
 namespace rtc {
 class AtomicOps {
@@ -31,10 +35,10 @@ class AtomicOps {
   static int Decrement(volatile int* i) {
     return ::InterlockedDecrement(reinterpret_cast<volatile LONG*>(i));
   }
-  static int Load(volatile const int* i) {
+  static int AcquireLoad(volatile const int* i) {
     return *i;
   }
-  static void Store(volatile int* i, int value) {
+  static void ReleaseStore(volatile int* i, int value) {
     *i = value;
   }
   static int CompareAndSwap(volatile int* i, int old_value, int new_value) {
@@ -49,14 +53,32 @@ class AtomicOps {
   static int Decrement(volatile int* i) {
     return __sync_sub_and_fetch(i, 1);
   }
-  static int Load(volatile const int* i) {
-    // Adding 0 is a no-op, so const_cast is fine.
-    return __sync_add_and_fetch(const_cast<volatile int*>(i), 0);
+#if defined(THREAD_SANITIZER)
+  // TSan doesn't understand the __sync_synchronize barrier used below. These
+  // versions are supplied to prevent false-positive data races.
+  static int32_t AcquireLoad(volatile const int32_t* i) {

[Alexander Potapenko, 2015/07/22 12:57:37]

It's better for TSan and non-TSan versions of these functions to have matching
prototypes.

[pbos-webrtc, 2015/07/22 13:04:40]

I need the compiler to pick 32/64 bit depending on platform, or are you saying
that I can assume the size of an int when building under TSan?

[Alexander Potapenko, 2015/07/22 13:10:08]

Yeah, I was saying the latter. Or you can explicitly convert int to int32_t in
the call to __tsan_atomic32_load():

  static int AcquireLoad(volatile const int* i) {
    return __tsan_atomic32_load((int32_t *)i, __tsan_memory_order_acquire);
  }

+    return __tsan_atomic32_load(i, __tsan_memory_order_acquire);
   }
-  static void Store(volatile int* i, int value) {
+  static int64_t AcquireLoad(volatile const int64_t* i) {
+    return __tsan_atomic64_load(i, __tsan_memory_order_acquire);
+  }
+  static void ReleaseStore(volatile int32_t* i, int32_t value) {
+    __tsan_atomic32_store(i, value, __tsan_memory_order_release);
+  }
+  static void ReleaseStore(volatile int64_t* i, int64_t value) {
+    __tsan_atomic64_store(i, value, __tsan_memory_order_release);
+  }
+#else
+  static int AcquireLoad(volatile const int* i) {
+    int value = *i;
+    __sync_synchronize();
+    return value;
+  }
+  static void ReleaseStore(volatile int* i, int value) {
     __sync_synchronize();
     *i = value;
   }
+#endif
   static int CompareAndSwap(volatile int* i, int old_value, int new_value) {
     return __sync_val_compare_and_swap(i, old_value, new_value);
   }