Fix memory ordering for EventBase::isRunning().

Summary: Technically speaking the contract of isRunning() is unclear on whether it can be used as a synchronization mechanism; it could, e.g. be used solely for diagnostics purposes, in which case memory_order_relaxed would be enough. But it is used at least to implement waitUntilRunning(), so memory_order_acquire/memory_order_release is needed to achieve proper synchronization.

Reviewed By: yfeldblum

Differential Revision: D26436697

fbshipit-source-id: 2be07c473e1cb286e6044fa677304816921e888d

folly/io/async/EventBase.cpp

@@ -286,7 +286,7 @@ static std::chrono::milliseconds getTimeDelta(
 }
 
 void EventBase::waitUntilRunning() {
-  while (!isRunning()) {
+  while (loopThread_.load(std::memory_order_acquire) == std::thread::id()) {
     std::this_thread::yield();
   }
 }
@@ -340,7 +340,7 @@ bool EventBase::loopBody(int flags, bool ignoreKeepAlive) {
   std::chrono::microseconds idle;
 
   auto const prevLoopThread = loopThread_.exchange(
-      std::this_thread::get_id(), std::memory_order_relaxed);
+      std::this_thread::get_id(), std::memory_order_release);
   CHECK_EQ(std::thread::id(), prevLoopThread)
       << "Driving an EventBase in one thread (" << std::this_thread::get_id()
       << ") while it is already being driven in another thread ("

folly/io/async/EventBase.h

@@ -663,7 +663,10 @@ class EventBase : public TimeoutManager,
   }
 
   /**
-   * check if the event base loop is running.
+   * Check if the event base loop is running.
+   *
+   * This may only be used as a sanity check mechanism; it cannot be used to
+   * make any decisions; for that, consider waitUntilRunning().
    */
   bool isRunning() const {
     return loopThread_.load(std::memory_order_relaxed) != std::thread::id();