X-Git-Url: https://git.lttng.org/?a=blobdiff_plain;f=src%2Fcommon%2Ffutex.cpp;h=b0e01fc73d4d8d4a6cbec6e669958fe80fa94b5e;hb=HEAD;hp=3eadf096bc8086760e36b24f3a342341d048c9fa;hpb=21cf9b6b1843774306a76f4dccddddd706b64f79;p=lttng-tools.git

diff --git a/src/common/futex.cpp b/src/common/futex.cpp
index 3eadf096b..b0e01fc73 100644
--- a/src/common/futex.cpp
+++ b/src/common/futex.cpp
@@ -7,15 +7,15 @@
  */
 
 #define _LGPL_SOURCE
+#include "futex.hpp"
+
+#include <common/common.hpp>
+
 #include <limits.h>
 #include <unistd.h>
 #include <urcu.h>
 #include <urcu/futex.h>
 
-#include <common/common.h>
-
-#include "futex.h"
-
 /*
  * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
  * "nto1" added to all function signature.
@@ -42,8 +42,7 @@ void futex_wait_update(int32_t *futex, int active)
 {
 	if (active) {
 		uatomic_set(futex, 1);
-		if (futex_async(futex, FUTEX_WAKE,
-				INT_MAX, NULL, NULL, 0) < 0) {
+		if (futex_async(futex, FUTEX_WAKE, INT_MAX, nullptr, nullptr, 0) < 0) {
 			PERROR("futex_async");
 			abort();
 		}
@@ -72,16 +71,25 @@ void futex_nto1_wait(int32_t *futex)
 {
 	cmm_smp_mb();
 
-	if (uatomic_read(futex) != -1)
-		goto end;
-	while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
+	while (uatomic_read(futex) == -1) {
+		if (!futex_async(futex, FUTEX_WAIT, -1, nullptr, nullptr, 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. */
 			PERROR("futex_async");
@@ -100,7 +108,7 @@ void futex_nto1_wake(int32_t *futex)
 	if (caa_unlikely(uatomic_read(futex) != -1))
 		goto end;
 	uatomic_set(futex, 0);
-	if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
+	if (futex_async(futex, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) {
 		PERROR("futex_async");
 		abort();
 	}