dm mpath: fix ioctl deadlock when no paths

When multipath needs to retry an ioctl the reference to the
current live table needs to be dropped. Otherwise a deadlock
occurs when all paths are down:
- dm_blk_ioctl takes a reference to the current table
and spins in multipath_ioctl().
- A new table is being loaded, but upon resume the process
hangs in dm_table_destroy() waiting for references to
drop to zero.

With this patch the reference to the old table is dropped
prior to retry, thereby avoiding the deadlock.

Signed-off-by: Hannes Reinecke <[e-mail]>
Cc: Mike Snitzer <[e-mail]>
Cc: [e-mail]
Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm-mpath.c

···
 	unsigned long flags;
 	int r;
 
-again:
 	bdev = NULL;
 	mode = 0;
 	r = 0;
···
 	}
 
 	if ((pgpath && m->queue_io) || (!pgpath && m->queue_if_no_path))
-		r = -EAGAIN;
+		r = -ENOTCONN;
 	else if (!bdev)
 		r = -EIO;
 
···
 	if (!r && ti->len != i_size_read(bdev->bd_inode) >> SECTOR_SHIFT)
 		r = scsi_verify_blk_ioctl(NULL, cmd);
 
-	if (r == -EAGAIN && !fatal_signal_pending(current)) {
+	if (r == -ENOTCONN && !fatal_signal_pending(current))
 		queue_work(kmultipathd, &m->process_queued_ios);
-		msleep(10);
-		goto again;
-	}
 
 	return r ? : __blkdev_driver_ioctl(bdev, mode, cmd, arg);
 }

drivers/md/dm.c

···
 			unsigned int cmd, unsigned long arg)
 {
 	struct mapped_device *md = bdev->bd_disk->private_data;
-	struct dm_table *map = dm_get_live_table(md);
+	struct dm_table *map;
 	struct dm_target *tgt;
 	int r = -ENOTTY;
 
+retry:
+	map = dm_get_live_table(md);
 	if (!map || !dm_table_get_size(map))
 		goto out;
 
···
 
 out:
 	dm_table_put(map);
+
+	if (r == -ENOTCONN) {
+		msleep(10);
+		goto retry;
+	}
 
 	return r;
 }