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 --- kernel/futex.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c index 60be2d3cfdd7..5677fbb97aea 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1553,11 +1553,7 @@ 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 @@ -1566,6 +1562,13 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q) * 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); }
/*