Fix concurrent access to VMState::current_state_.

The main fix is for current_state() and external_callback() accessors.
I also applied memory access ordering on current_state_ modification,
mainly to reflect the fact that it is being shared among VM and profiler
sampler threads.

BUG=361

Review URL: http://codereview.chromium.org/2852047

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5035 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/vm-state-inl.h

@@ -74,8 +74,10 @@ VMState::VMState(StateTag state)
   if (state == EXTERNAL) state = OTHER;
 #endif
   state_ = state;
-  previous_ = current_state_;  // Save the previous state.
-  current_state_ = this;       // Install the new state.
+  // Save the previous state.
+  previous_ = reinterpret_cast<VMState*>(current_state_);
+  // Install the new state.
+  OS::ReleaseStore(&current_state_, reinterpret_cast<AtomicWord>(this));
 
 #ifdef ENABLE_LOGGING_AND_PROFILING
   if (FLAG_log_state_changes) {
@@ -103,7 +105,8 @@ VMState::VMState(StateTag state)
 
 VMState::~VMState() {
   if (disabled_) return;
-  current_state_ = previous_;  // Return to the previous state.
+  // Return to the previous state.
+  OS::ReleaseStore(&current_state_, reinterpret_cast<AtomicWord>(previous_));
 
 #ifdef ENABLE_LOGGING_AND_PROFILING
   if (FLAG_log_state_changes) {

src/vm-state.cc

@@ -33,7 +33,7 @@ namespace v8 {
 namespace internal {
 
 #ifdef ENABLE_VMSTATE_TRACKING
-VMState* VMState::current_state_ = NULL;
+AtomicWord VMState::current_state_ = 0;
 #endif
 
 } }  // namespace v8::internal

src/vm-state.h

@@ -44,15 +44,17 @@ class VMState BASE_EMBEDDED {
 
   // Used for debug asserts.
   static bool is_outermost_external() {
-    return current_state_ == NULL;
+    return current_state_ == 0;
   }
 
   static StateTag current_state() {
-    return current_state_ ? current_state_->state() : EXTERNAL;
+    VMState* state = reinterpret_cast<VMState*>(current_state_);
+    return state ? state->state() : EXTERNAL;
   }
 
   static Address external_callback() {
-    return current_state_ ? current_state_->external_callback_ : NULL;
+    VMState* state = reinterpret_cast<VMState*>(current_state_);
+    return state ? state->external_callback_ : NULL;
   }
 
  private:
@@ -62,7 +64,7 @@ class VMState BASE_EMBEDDED {
   Address external_callback_;
 
   // A stack of VM states.
-  static VMState* current_state_;
+  static AtomicWord current_state_;
 #else
  public:
   explicit VMState(StateTag state) {}