quota: optimize mark_dirty logic

- Skip locking if quota is dirty already.
- Return old quota state to help fs-specciffic implementation to optimize
  case where quota was dirty already.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/dquot.c

@@ -317,14 +317,23 @@ static inline int mark_dquot_dirty(struct dquot *dquot)
 	return dquot->dq_sb->dq_op->mark_dirty(dquot);
 }
 
+/* Mark dquot dirty in atomic manner, and return it's old dirty flag state */
 int dquot_mark_dquot_dirty(struct dquot *dquot)
 {
+	int ret = 1;
+
+	/* If quota is dirty already, we don't have to acquire dq_list_lock */
+	if (test_bit(DQ_MOD_B, &dquot->dq_flags))
+		return 1;
+
 	spin_lock(&dq_list_lock);
-	if (!test_and_set_bit(DQ_MOD_B, &dquot->dq_flags))
+	if (!test_and_set_bit(DQ_MOD_B, &dquot->dq_flags)) {
 		list_add(&dquot->dq_dirty, &sb_dqopt(dquot->dq_sb)->
 				info[dquot->dq_type].dqi_dirty_list);
+		ret = 0;
+	}
 	spin_unlock(&dq_list_lock);
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL(dquot_mark_dquot_dirty);