GFS2: Fix race between lru_list and glock ref count

This patch fixes a narrow race window between the glock ref count
hitting zero and glocks being removed from the lru_list.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/glock.c

@@ -167,14 +167,19 @@ void gfs2_glock_add_to_lru(struct gfs2_glock *gl)
 	spin_unlock(&lru_lock);
 }
 
-static void gfs2_glock_remove_from_lru(struct gfs2_glock *gl)
+static void __gfs2_glock_remove_from_lru(struct gfs2_glock *gl)
 {
-	spin_lock(&lru_lock);
 	if (!list_empty(&gl->gl_lru)) {
 		list_del_init(&gl->gl_lru);
 		atomic_dec(&lru_count);
 		clear_bit(GLF_LRU, &gl->gl_flags);
 	}
+}
+
+static void gfs2_glock_remove_from_lru(struct gfs2_glock *gl)
+{
+	spin_lock(&lru_lock);
+	__gfs2_glock_remove_from_lru(gl);
 	spin_unlock(&lru_lock);
 }
 
@@ -217,11 +222,12 @@ void gfs2_glock_put(struct gfs2_glock *gl)
 	struct gfs2_sbd *sdp = gl->gl_sbd;
 	struct address_space *mapping = gfs2_glock2aspace(gl);
 
-	if (atomic_dec_and_test(&gl->gl_ref)) {
+	if (atomic_dec_and_lock(&gl->gl_ref, &lru_lock)) {
+		__gfs2_glock_remove_from_lru(gl);
+		spin_unlock(&lru_lock);
 		spin_lock_bucket(gl->gl_hash);
 		hlist_bl_del_rcu(&gl->gl_list);
 		spin_unlock_bucket(gl->gl_hash);
-		gfs2_glock_remove_from_lru(gl);
 		GLOCK_BUG_ON(gl, !list_empty(&gl->gl_holders));
 		GLOCK_BUG_ON(gl, mapping && mapping->nrpages);
 		trace_gfs2_glock_put(gl);