linux-kernel - [PATCH mm] epoll: lock ep->mtx in ep

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Message-ID: <20130314030900.GA30154@dcvr.yhbt.net>
Date:	Thu, 14 Mar 2013 03:09:00 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	NeilBrown <neilb@...e.de>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH mm] epoll: lock ep->mtx in ep_free to silence lockdep

Technically we do not need to hold ep->mtx during ep_free since we are
certain there are no other users of ep at that point.  However, lockdep
complains with a "suspicious rcu_dereference_check() usage!" message;
so lock the mutex before ep_remove to silence the warning.

Signed-off-by: Eric Wong <normalperson@...t.net>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arve Hjønnevåg <arve@...roid.com>
Cc: Davide Libenzi <davidel@...ilserver.org>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: NeilBrown <neilb@...e.de>,
Cc: Rafael J. Wysocki <rjw@...k.pl>
---
  I considered using lockdep_off()/lockdep_on(), but I figure that may
  hide other bugs...

 fs/eventpoll.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 633e69f..d71b754 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -745,11 +745,15 @@ static void ep_free(struct eventpoll *ep)
 	 * point we are sure no poll callbacks will be lingering around, and also by
 	 * holding "epmutex" we can be sure that no file cleanup code will hit
 	 * us during this operation. So we can avoid the lock on "ep->lock".
+	 * We do not need to lock ep->mtx, either, we only do it to prevent
+	 * a lockdep warning.
 	 */
+	mutex_lock(&ep->mtx);
 	while ((rbp = rb_first(&ep->rbr)) != NULL) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		ep_remove(ep, epi);
 	}
+	mutex_unlock(&ep->mtx);
 
 	mutex_unlock(&epmutex);
 	mutex_destroy(&ep->mtx);
-- 
Eric Wong
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