linux-kernel - [PATCH 3/4] signalfd: signalfd_cleanup() can race with remove_wait

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Date:	Wed, 22 Feb 2012 18:34:42 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Greg KH <greg@...ah.com>, Jason Baron <jbaron@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...k.frob.com>
Cc:	Eugene Teo <eugeneteo@...nel.sg>, Maxime Bizon <mbizon@...ebox.fr>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/4] signalfd: signalfd_cleanup() can race with
	remove_wait_queue()

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@...ebox.fr>
Cc: <stable@...nel.org>
Signed-off-by: Oleg Nesterov <oleg@...hat.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


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