[PATCH 3/4] signalfd: signalfd_cleanup() can race with remove_wait_queue()

[Oleg Nesterov <oleg@redhat.com>]

signalfd_cleanup() checks waitqueue_active() lockless, this can
race with ep_unregister_pollwait(). We can see list_empty() == T
before remove_wait_queue() completes and list_empty_careful()
can't help. Add spin_unlock_wait() to serialize.

Reported-by: Maxime Bizon <mbizon@freebox.fr>
Cc: <stable@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/signalfd.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/fs/signalfd.c b/fs/signalfd.c
index 838ba21..6e51887 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -34,6 +34,10 @@ void signalfd_cleanup(struct sighand_struct *sighand)
 {
 	wait_queue_head_t *wqh = &sighand->signalfd_wqh;
 
+	/* make sure we can't race with remove_wait_queue() in progress */
+	spin_unlock_wait(&wqh->lock);
+	smp_rmb();
+
 	if (likely(!waitqueue_active(wqh)))
 		return;
 
-- 
1.5.5.1