quota: Do not acquire dqio_sem for dquot overwrites in v2 format

When dquot has space already allocated in a quota file, we just
overwrite that place when writing dquot. So we don't need any protection
against other modifications of quota file as these keep dquot in place.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/quota_v2.c

@@ -300,12 +300,23 @@ static int v2_write_dquot(struct dquot *dquot)
 {
 	struct quota_info *dqopt = sb_dqopt(dquot->dq_sb);
 	int ret;
+	bool alloc = false;
 
-	down_write(&dqopt->dqio_sem);
+	/*
+	 * If space for dquot is already allocated, we don't need any
+	 * protection as we'll only overwrite the place of dquot. We are
+	 * still protected by concurrent writes of the same dquot by
+	 * dquot->dq_lock.
+	 */
+	if (!dquot->dq_off) {
+		alloc = true;
+		down_write(&dqopt->dqio_sem);
+	}
 	ret = qtree_write_dquot(
 			sb_dqinfo(dquot->dq_sb, dquot->dq_id.type)->dqi_priv,
 			dquot);
-	up_write(&dqopt->dqio_sem);
+	if (alloc)
+		up_write(&dqopt->dqio_sem);
 	return ret;
 }