Fix: waiter: futex wait: handle spurious futex wakeups

[lttng-tools.git] / src / common / waiter.cpp

diff --git a/src/common/waiter.cpp b/src/common/waiter.cpp
index 3ddb68feb7c76dd91f09bec9d7664c02224627a2..2a1dded89805a6188a5b77b9f75e8ce5a99a976c 100644 (file)
--- a/src/common/waiter.cpp
+++ b/src/common/waiter.cpp
@@ -49,15 +49,25 @@ void lttng_waiter_wait(struct lttng_waiter *waiter)
                }
                caa_cpu_relax();
        }
-       while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
-                       NULL, NULL, 0)) {
+       while (uatomic_read(&waiter->state) == WAITER_WAITING) {
+               if (!futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_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
+                        * WAITER_WAITING (spurious wakeups). Check
+                        * the value again in user-space to validate
+                        * whether it really differs from WAITER_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. */
                        PERROR("futex_noasync");