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, 12 insertions, 12 deletions

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index b2e26e2120..f81fca5c11 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -428,17 +428,17 @@ void qemu_event_wait(QemuEvent *ev)
 
     assert(ev->initialized);
 
-    /*
-     * qemu_event_wait must synchronize with qemu_event_set even if it does
-     * not go down the slow path, so this load-acquire is needed that
-     * synchronizes with the first memory barrier in qemu_event_set().
-     *
-     * If we do go down the slow path, there is no requirement at all: we
-     * might miss a qemu_event_set() here but ultimately the memory barrier in
-     * qemu_futex_wait() will ensure the check is done correctly.
-     */
-    value = qatomic_load_acquire(&ev->value);
-    if (value != EV_SET) {
+    while (true) {
+        /*
+         * qemu_event_wait must synchronize with qemu_event_set even if it does
+         * not go down the slow path, so this load-acquire is needed that
+         * synchronizes with the first memory barrier in qemu_event_set().
+         */
+        value = qatomic_load_acquire(&ev->value);
+        if (value == EV_SET) {
+            break;
+        }
+
         if (value == EV_FREE) {
             /*
              * Leave the event reset and tell qemu_event_set that there are
@@ -452,7 +452,7 @@ void qemu_event_wait(QemuEvent *ev)
              * like the load above.
              */
             if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
-                return;
+                break;
             }
         }