nfsd: Fix a soft lockup race in nfsd_file_mark_find_or_create()

If nfsd_file_mark_find_or_create() keeps winning the race for the
nfsd_file_fsnotify_group->mark_mutex against nfsd_file_mark_put()
then it can soft lock up, since fsnotify_add_inode_mark() ends
up always finding an existing entry.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/filecache.c

@@ -132,9 +132,13 @@ nfsd_file_mark_find_or_create(struct nfsd_file *nf)
 						 struct nfsd_file_mark,
 						 nfm_mark));
 			mutex_unlock(&nfsd_file_fsnotify_group->mark_mutex);
-			fsnotify_put_mark(mark);
-			if (likely(nfm))
+			if (nfm) {
+				fsnotify_put_mark(mark);
 				break;
+			}
+			/* Avoid soft lockup race with nfsd_file_mark_put() */
+			fsnotify_destroy_mark(mark, nfsd_file_fsnotify_group);
+			fsnotify_put_mark(mark);
 		} else
 			mutex_unlock(&nfsd_file_fsnotify_group->mark_mutex);