[PATCH mm] epoll: lock ep->mtx in ep_free to silence lockdep

From: Eric Wong
Date: Wed Mar 13 2013 - 23:09:10 EST

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@xxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arve HjÃnnevÃg <arve@xxxxxxxxxxx>
Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: NeilBrown <neilb@xxxxxxx>,
Cc: Rafael J. Wysocki <rjw@xxxxxxx>
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);

Eric Wong
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