futex: Fix (possible) missed wakeup

We must not rely on wake_q_add() to delay the wakeup; in particular
commit:

  1d0dcb3ad9d3 ("futex: Implement lockless wakeups")

moved wake_q_add() before smp_store_release(&q->lock_ptr, NULL), which
could result in futex_wait() waking before observing ->lock_ptr ==
NULL and going back to sleep again.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/futex.c b/kernel/futex.c
index be3bff2315ff75c46565a42c126f05641c753b98..fdd312da09927ad43e2b6dabd7b089d3c2e8393b 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1452,11 +1452,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
        if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
                return;
 
-       /*
-        * Queue the task for later wakeup for after we've released
-        * the hb->lock. wake_q_add() grabs reference to p.
-        */
-       wake_q_add(wake_q, p);
+       get_task_struct(p);
        __unqueue_futex(q);
        /*
         * The waiting task can free the futex_q as soon as q->lock_ptr = NULL
@@ -1466,6 +1462,13 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
         * plist_del in __unqueue_futex().
         */
        smp_store_release(&q->lock_ptr, NULL);
+
+       /*
+        * Queue the task for later wakeup for after we've released
+        * the hb->lock. wake_q_add() grabs reference to p.
+        */
+       wake_q_add(wake_q, p);
+       put_task_struct(p);
 }
 
 /*