Baton and Futex: Add comments about async-signal-safety

Summary: Add comments stating the expectations for Baton post and Futex wake native implementation to be async-signal-safe.

Reviewed By: ot, luciang

Differential Revision: D33461663

fbshipit-source-id: 2855427cc94f577b566b19203a4523f54a41d0aa

folly/detail/Futex.cpp

@@ -40,6 +40,8 @@ namespace {
 ////////////////////////////////////////////////////
 // native implementation using the futex() syscall
 
+// The native implementation of futex wake must be async-signal-safe.
+
 #ifdef __linux__
 
 /// Certain toolchains (like Android's) don't include the full futex API in
@@ -161,6 +163,8 @@ FutexResult nativeFutexWaitImpl(
 ///////////////////////////////////////////////////////
 // compatibility implementation using standard C++ API
 
+// This implementation may be non-async-signal-safe.
+
 using Lot = ParkingLot<uint32_t>;
 Lot parkingLot;
 

folly/synchronization/Baton.h

@@ -55,6 +55,11 @@ namespace folly {
 /// DeterministicSchedule.  By having a much more restrictive
 /// lifecycle we can also add a bunch of assertions that can help to
 /// catch race conditions ahead of time.
+///
+/// Baton post with MayBlock == false is async-signal-safe.
+/// When MayBlock == true, Baton post is async-signal-safe if
+/// Futex wake is so.
+
 template <bool MayBlock = true, template <typename> class Atom = std::atomic>
 class Baton {
  public: