[PATCH] fix race in inotify_release

[Amy Griffis <amy.griffis@hp.com>]

While doing some inotify stress testing, I hit the following race.  In
inotify_release(), it's possible for a watch to be removed from the
lists in between dropping dev->mutex and taking inode->inotify_mutex.
The reference we hold prevents the watch from being freed, but not
from being removed.

Checking the dev's idr mapping will prevent a double list_del of the
same watch.

Signed-off-by: Amy Griffis <amy.griffis@hp.com>

diff --git a/fs/inotify.c b/fs/inotify.c
index 1f50302..7d57253 100644
--- a/fs/inotify.c
+++ b/fs/inotify.c
@@ -848,7 +848,11 @@ static int inotify_release(struct inode 
 		inode = watch->inode;
 		mutex_lock(&inode->inotify_mutex);
 		mutex_lock(&dev->mutex);
-		remove_watch_no_event(watch, dev);
+
+		/* make sure we didn't race with another list removal */
+		if (likely(idr_find(&dev->idr, watch->wd)))
+			remove_watch_no_event(watch, dev);
+
 		mutex_unlock(&dev->mutex);
 		mutex_unlock(&inode->inotify_mutex);
 		put_inotify_watch(watch);