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>

1 files changed, 8 insertions, 5 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index be3bff2315ff..fdd312da0992 100644
--- 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);
 }
 
 /*