event: ensure acquire release semantics for muxnote disposal

While it is sufficient to use the relaxed ordering for the acquire semantics in the `_dispatch_muxnote_retain`, we need to use the acquire release semantics on the release in `_dispatch_muxnote_release` to ensure that any pending retains are not interrupted. This should hopefully alleviate the occasional crashes that have been observed with the muxnote reference counting. Fixes: #887, #844

Author: compnerd

src/event/event_windows.c

@@ -260,8 +260,16 @@ _dispatch_muxnote_retain(dispatch_muxnote_t dmn)
 static void
 _dispatch_muxnote_release(dispatch_muxnote_t dmn)
 {
-uintptr_t refcount = os_atomic_dec(&dmn->dmn_refcount, relaxed);
+// We perform a minor optimization here - perform the decrement with
+// release semantics. In the case that we are going to dispose of the
+// value, we perform the acquire fence. This reduces the cost on the
+// normal path by avoiding the acquire fence. This should be more
+// beneficial on ARM64, as X64 being TSO'ed doesn't gain much. However,
+// `mfence` being isolated should hopefully be a bit more efficient than
+// the repeated `lock` if there is contention.
+uintptr_t refcount = os_atomic_dec(&dmn->dmn_refcount, release);
 if (refcount == 0) {
+os_atomic_thread_fence(acquire);
 _dispatch_muxnote_dispose(dmn);
 } else if (refcount == UINTPTR_MAX) {
 DISPATCH_INTERNAL_CRASH(0, "muxnote refcount underflow");