Fix: waiter: futex wait: handle spurious futex wakeups

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

diff --git a/src/common/waiter.c b/src/common/waiter.c
index c5a2829408215784952d047e272c12682e291c23..db2de6557f5df5a5465d66c8e2df591202953983 100644 (file)
--- a/src/common/waiter.c
+++ b/src/common/waiter.c
@@ -1,21 +1,9 @@
 /*
- * Copyright (C) 2012 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
- *               2017 - Jérémie Galarneau <jeremie.galarneau@efficios.com>
+ * Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com>
  *
- * This program is free software; you can redistribute it and/or modify it
- * under the terms of the GNU Lesser General Public License, version 2.1 only,
- * as published by the Free Software Foundation.
+ * SPDX-License-Identifier: LGPL-2.1-only
  *
- * This program is distributed in the hope that it will be useful, but WITHOUT
- * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
- * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
- * for more details.
- *
- * You should have received a copy of the GNU Lesser General Public License
- * along with this program; if not, write to the Free Software Foundation,
- * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
- *
- * This code is originally adapted from userspace-rcu's urcu-wait.h
  */
 
 #include "waiter.h"
@@ -64,15 +52,25 @@ 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");