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 94f3e3559ba8c32492234f911911db7faa1337a9..a7195be0c91266a6224ce86d73a4811c8695952d 100644 (file)
--- a/src/urcu-wait.h
+++ b/src/urcu-wait.h
@@ -60,8 +60,13 @@ struct urcu_wait_queue {
        struct cds_wfs_stack stack;
 };
 
-#define URCU_WAIT_QUEUE_HEAD_INIT(name)                        \
-       { .stack.head = CDS_WFS_END, .stack.lock = PTHREAD_MUTEX_INITIALIZER }
+#define URCU_WAIT_QUEUE_HEAD_INIT(name)                                \
+       {                                                       \
+                .stack = {                                     \
+                       .head = CDS_WFS_END,                    \
+                       .lock = PTHREAD_MUTEX_INITIALIZER,      \
+               },                                              \
+       }
 
 #define DECLARE_URCU_WAIT_QUEUE(name)                  \
        struct urcu_wait_queue name
@@ -148,15 +153,26 @@ 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);