futex: Check value after qemu_futex_wait()

futex(2) - Linux manual page
https://man7.org/linux/man-pages/man2/futex.2.html
> Note that a wake-up can also be caused by common futex usage patterns
> in unrelated code that happened to have previously used the futex
> word's memory location (e.g., typical futex-based implementations of
> Pthreads mutexes can cause this under some conditions).  Therefore,
> callers should always conservatively assume that a return value of 0
> can mean a spurious wake-up, and use the futex word's value (i.e.,
> the user-space synchronization scheme) to decide whether to continue
> to block or not.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Link: https://lore.kernel.org/r/20250529-event-v5-1-53b285203794@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/tests/unit/test-aio-multithread.c b/tests/unit/test-aio-multithread.c
index 08d4570ccb..8c2e41545a 100644
--- a/tests/unit/test-aio-multithread.c
+++ b/tests/unit/test-aio-multithread.c
@@ -305,7 +305,9 @@ static void mcs_mutex_lock(void)
     prev = qatomic_xchg(&mutex_head, id);
     if (prev != -1) {
         qatomic_set(&nodes[prev].next, id);
-        qemu_futex_wait(&nodes[id].locked, 1);
+        while (qatomic_read(&nodes[id].locked) == 1) {
+            qemu_futex_wait(&nodes[id].locked, 1);
+        }
     }
 }