[PATCH 4.9 44/53] futex: Fix (possible) missed wakeup

Greg Kroah-Hartman
2021-03-29T08:05:38Z

From: Peter Zijlstra <peterz@infradead.org>

commit b061c38bef43406df8e73c5be06cbfacad5ee6ad upstream.

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/futex.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1553,11 +1553,7 @@ static void mark_wake_futex(struct wake_
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
@@ -1566,6 +1562,13 @@ static void mark_wake_futex(struct wake_
* store to lock_ptr from getting ahead of the plist_del.
*/
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);
}

/*