Fix: urcu-wait: futex wait: handle spurious futex wakeups

[urcu.git] / src / urcu-wait.h

diff --git a/src/urcu-wait.h b/src/urcu-wait.h
index c586ec9da8a007519cc7d6a793a9b6598cea46be..ef5f7ede318c2b3f0aaddcde7aec189aa1bccf60 100644 (file)
--- a/src/urcu-wait.h
+++ b/src/urcu-wait.h
@@ -154,15 +154,26 @@ void urcu_adaptative_busy_wait(struct urcu_wait_node *wait)
                        goto skip_futex_wait;
                caa_cpu_relax();
        }
-       while (futex_noasync(&wait->state, FUTEX_WAIT, URCU_WAIT_WAITING,
-                       NULL, NULL, 0)) {
+       while (uatomic_read(&wait->state) == URCU_WAIT_WAITING) {
+               if (!futex_noasync(&wait->state, FUTEX_WAIT, URCU_WAIT_WAITING, NULL, NULL, 0)) {
+                       /*
+                        * Prior queued wakeups queued by unrelated code
+                        * using the same address can cause futex wait to
+                        * return 0 even through the futex value is still
+                        * URCU_WAIT_WAITING (spurious wakeups). Check
+                        * the value again in user-space to validate
+                        * whether it really differs from
+                        * URCU_WAIT_WAITING.
+                        */
+                       continue;
+               }
                switch (errno) {
-               case EWOULDBLOCK:
+               case EAGAIN:
                        /* Value already changed. */
                        goto skip_futex_wait;
                case EINTR:
                        /* Retry if interrupted by signal. */
-                       break;  /* Get out of switch. */
+                       break;  /* Get out of switch. Check again. */
                default:
                        /* Unexpected error. */
                        urcu_die(errno);