fsnotify: don't BUG in fsnotify_destroy_mark()

Removing the parent of a watched file results in "kernel BUG at
fs/notify/mark.c:139".

To reproduce

  add "-w /tmp/audit/dir/watched_file" to audit.rules
  rm -rf /tmp/audit/dir

This is caused by fsnotify_destroy_mark() being called without an
extra reference taken by the caller.

Reported by Francesco Cosoleto here:

  https://bugzilla.novell.com/show_bug.cgi?id=689860

Fix by removing the BUG_ON and adding a comment about not accessing mark after
the iput.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index e14587d55689..f104d565b682 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -135,9 +135,6 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark)
 
         mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 
-        /* 1 from caller and 1 for being on i_list/g_list */
-        BUG_ON(atomic_read(&mark->refcnt) < 2);
-
         spin_lock(&group->mark_lock);
 
         if (mark->flags & FSNOTIFY_MARK_FLAG_INODE) {
@@ -182,6 +179,11 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark)
                 iput(inode);
 
         /*
+         * We don't necessarily have a ref on mark from caller so the above iput
+         * may have already destroyed it.  Don't touch from now on.
+         */
+
+        /*
          * it's possible that this group tried to destroy itself, but this
          * this mark was simultaneously being freed by inode.  If that's the
          * case, we finish freeing the group here.