quota: fix dquot_disable vs dquot_transfer race v2

I've got following lockup:
dquot_disable                              dquot_transfer
                                            ->dqget()
					       sb_has_quota_active
dqopt->flags &= ~dquot_state_flag(f, cnt)      atomic_inc(dq->dq_count)
 ->drop_dquot_ref(sb, cnt);
    down_write(dqptr_sem)
    inode->i_dquot[cnt] = NULL              ->__dquot_transfer
invalidate_dquots(sb, cnt);		       down_write(&dqptr_sem)
  ->wait for dq_wait_unused		       inode->i_dquot = new_dquot
  /* wait forever */                            ^^^^New quota user^^^^^^

We cannot allow new references to dquots from inodes after drop_dquot_ref()
has removed them.  We have to recheck quota state under dqptr_sem and before
assignment, as we do it in dquot_initialize().

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

fs/quota/dquot.c

@@ -1736,6 +1736,7 @@ int __dquot_transfer(struct inode *inode, struct dquot **transfer_to)
 	qsize_t rsv_space = 0;
 	struct dquot *transfer_from[MAXQUOTAS] = {};
 	int cnt, ret = 0;
+	char is_valid[MAXQUOTAS] = {};
 	char warntype_to[MAXQUOTAS];
 	char warntype_from_inodes[MAXQUOTAS], warntype_from_space[MAXQUOTAS];
 
@@ -1757,8 +1758,15 @@ int __dquot_transfer(struct inode *inode, struct dquot **transfer_to)
 	space = cur_space + rsv_space;
 	/* Build the transfer_from list and check the limits */
 	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
+		/*
+		 * Skip changes for same uid or gid or for turned off quota-type.
+		 */
 		if (!transfer_to[cnt])
 			continue;
+		/* Avoid races with quotaoff() */
+		if (!sb_has_quota_active(inode->i_sb, cnt))
+			continue;
+		is_valid[cnt] = 1;
 		transfer_from[cnt] = inode->i_dquot[cnt];
 		ret = check_idq(transfer_to[cnt], 1, warntype_to + cnt);
 		if (ret)
@@ -1772,12 +1780,8 @@ int __dquot_transfer(struct inode *inode, struct dquot **transfer_to)
 	 * Finally perform the needed transfer from transfer_from to transfer_to
 	 */
 	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
-		/*
-		 * Skip changes for same uid or gid or for turned off quota-type.
-		 */
-		if (!transfer_to[cnt])
+		if (!is_valid[cnt])
 			continue;
-
 		/* Due to IO error we might not have transfer_from[] structure */
 		if (transfer_from[cnt]) {
 			warntype_from_inodes[cnt] =
@@ -1803,7 +1807,9 @@ int __dquot_transfer(struct inode *inode, struct dquot **transfer_to)
 	mark_all_dquot_dirty(transfer_to);
 	/* Pass back references to put */
 	for (cnt = 0; cnt < MAXQUOTAS; cnt++)
-		transfer_to[cnt] = transfer_from[cnt];
+		if (is_valid[cnt])
+			transfer_to[cnt] = transfer_from[cnt];
+
 warn:
 	flush_warnings(transfer_to, warntype_to);
 	flush_warnings(transfer_from, warntype_from_inodes);