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event: ensure acquire release semantics for muxnote disposal #898

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Merged
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Aug 15, 2025
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event: ensure acquire release semantics for muxnote disposal #898

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10 changes: 9 additions & 1 deletion src/event/event_windows.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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");
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