gfs2: Add missing rcu locking for glock lookup

BugLink: http://bugs.launchpad.net/bugs/1673538

commit f38e5fb9 upstream.

We must hold the rcu read lock across looking up glocks and trying to
bump their refcount to prevent the glocks from being freed in between.
Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

fs/gfs2/glock.c

@@ -651,9 +651,11 @@ int gfs2_glock_get(struct gfs2_sbd *sdp, u64 number,
 	struct kmem_cache *cachep;
 	int ret, tries = 0;
 
+	rcu_read_lock();
 	gl = rhashtable_lookup_fast(&gl_hash_table, &name, ht_parms);
 	if (gl && !lockref_get_not_dead(&gl->gl_lockref))
 		gl = NULL;
+	rcu_read_unlock();
 
 	*glp = gl;
 	if (gl)
@@ -721,15 +723,18 @@ int gfs2_glock_get(struct gfs2_sbd *sdp, u64 number,
 
 	if (ret == -EEXIST) {
 		ret = 0;
+		rcu_read_lock();
 		tmp = rhashtable_lookup_fast(&gl_hash_table, &name, ht_parms);
 		if (tmp == NULL || !lockref_get_not_dead(&tmp->gl_lockref)) {
 			if (++tries < 100) {
+				rcu_read_unlock();
 				cond_resched();
 				goto again;
 			}
 			tmp = NULL;
 			ret = -ENOMEM;
 		}
+		rcu_read_unlock();
 	} else {
 		WARN_ON_ONCE(ret);
 	}