Fix an atomic memory ordering in DefaultKeepAliveExecutor.

Summary: Honestly I haven't spent time to understand why this order needs to be acq_rel, but (1) it's the same as the one used in the other keepAliveRelease in this file (2) this ordering is free for read-modify-write operations (3) TSAN said this works :)

Reviewed By: yfeldblum

Differential Revision: D23495784

fbshipit-source-id: 8bdcafb14b27e6c0f8838ed478364821cd39f9a9

folly/DefaultKeepAliveExecutor.h

@@ -140,7 +140,7 @@ class DefaultKeepAliveExecutor : public virtual Executor {
 
   void keepAliveRelease() noexcept override {
     auto keepAliveCount =
-        controlBlock_->keepAliveCount_.fetch_sub(1, std::memory_order_acquire);
+        controlBlock_->keepAliveCount_.fetch_sub(1, std::memory_order_acq_rel);
     DCHECK(keepAliveCount >= 1);
 
     if (keepAliveCount == 1) {