X-Git-Url: http://git.lttng.org/?p=urcu.git;a=blobdiff_plain;f=src%2Furcu-qsbr.c;h=5572d39da65f1030510274b0618e301e893cfbcd;hp=3709412f2519ff7259da6bb144f9818584842580;hb=b1bf6ed74ea71db46ab9ab1ace3cac9de00bd380;hpb=1c7364b78b023245aa490007ce835a8ad530edf6

diff --git a/src/urcu-qsbr.c b/src/urcu-qsbr.c
index 3709412..5572d39 100644
--- a/src/urcu-qsbr.c
+++ b/src/urcu-qsbr.c
@@ -125,17 +125,25 @@ static void wait_gp(void)
 {
 	/* Read reader_gp before read futex */
 	cmm_smp_rmb();
-	if (uatomic_read(&urcu_qsbr_gp.futex) != -1)
-		return;
-	while (futex_noasync(&urcu_qsbr_gp.futex, FUTEX_WAIT, -1,
-			NULL, NULL, 0)) {
+	while (uatomic_read(&urcu_qsbr_gp.futex) == -1) {
+		if (!futex_noasync(&urcu_qsbr_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. */
 			return;
 		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);