Fix: urcu: futex wait: handle spurious futex wakeups

[urcu.git] / src / urcu.c

diff --git a/src/urcu.c b/src/urcu.c
index 2ebe993e33b5fdd3a540f3b15d7b92b450106766..a8c033a0bb7c960755428ec7a3caeff6b3f309e7 100644 (file)
--- a/src/urcu.c
+++ b/src/urcu.c
@@ -266,17 +266,25 @@ static void wait_gp(void)
        smp_mb_master();
        /* Temporarily unlock the registry lock. */
        mutex_unlock(&rcu_registry_lock);
-       if (uatomic_read(&rcu_gp.futex) != -1)
-               goto end;
-       while (futex_async(&rcu_gp.futex, FUTEX_WAIT, -1,
-                       NULL, NULL, 0)) {
+       while (uatomic_read(&rcu_gp.futex) == -1) {
+               if (!futex_async(&rcu_gp.futex, FUTEX_WAIT, -1, 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
+                        * -1 (spurious wakeups). Check the value again
+                        * in user-space to validate whether it really
+                        * differs from -1.
+                        */
+                       continue;
+               }
                switch (errno) {
-               case EWOULDBLOCK:
+               case EAGAIN:
                        /* Value already changed. */
                        goto end;
                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);
@@ -622,7 +630,9 @@ void alias_rcu_init(void);
 #endif
 
 #ifdef RCU_SIGNAL
-static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
+static void sigrcu_handler(int signo __attribute__((unused)),
+               siginfo_t *siginfo __attribute__((unused)),
+               void *context __attribute__((unused)))
 {
        /*
         * Executing this cmm_smp_mb() is the only purpose of this signal handler.