[libcxx-commits] [libcxx] [libc++] fix `counting_semaphore` lost wakeups (PR #79265)

[Jan Kokemüller via libcxx-commits]

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@@ -94,20 +95,25 @@ public:
     auto __old = __a_.fetch_add(__update, memory_order_release);
     _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
         __update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
-
-    if (__old > 0) {
-      // Nothing to do
-    } else if (__update > 1)
+    (void)__old;
+    if (__update > 1) {
       __a_.notify_all();
-    else
+    } else {
       __a_.notify_one();
+    }
   }
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() {
     auto const __test_fn = [this]() -> bool {
       auto __old = __a_.load(memory_order_relaxed);
       return (__old != 0) && __a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
     };
-    __cxx_atomic_wait(&__a_.__a_, __test_fn);
+    auto const __test_with_old = [this](__cxx_contention_t& __monitor) -> bool {
----------------
jiixyj wrote:

> when this function is called, it is possible that the atomic variable has already been changed and __monitor is the new value. so we are waiting for a different value now. (and also there is no happens-before so __test_fn could be evaluated before taking the monitor). so basically we need to fix this function.

Agreed! But I don't think the `__test_fn` can be reordered before taking the monitor, as taking the monitor is a "acquire" operation?

https://github.com/llvm/llvm-project/pull/79265