From: Hannes Roth <hannesr@fb.com>
Date: Thu, 25 Jun 2015 15:44:29 +0000 (-0700)
Subject: (Wangle) Fix possible race in updating FSM state
X-Git-Tag: v0.48.0~11
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=11e76315877d544604687b8823d75d525b74a781;p=folly.git

(Wangle) Fix possible race in updating FSM state

Summary: Storing the new state could be a memory race according to C++ (but wasn't in practice). I only checked GCC though.

Reviewed By: @nbronson

Differential Revision: D2189287
---

diff --git a/folly/futures/detail/FSM.h b/folly/futures/detail/FSM.h
index ce77c551..b706ed04 100644
--- a/folly/futures/detail/FSM.h
+++ b/folly/futures/detail/FSM.h
@@ -55,12 +55,12 @@ public:
     if (!mutex_.try_lock()) {
       mutex_.lock();
     }
-    if (state_.load(std::memory_order_relaxed) != A) {
+    if (state_.load(std::memory_order_acquire) != A) {
       mutex_.unlock();
       return false;
     }
     action();
-    state_.store(B, std::memory_order_relaxed);
+    state_.store(B, std::memory_order_release);
     mutex_.unlock();
     return true;
   }