Commit c8691cd0 authored Sep 13, 2024 by Mikulas Patocka

Revert "dm: requeue IO if mapping table not yet available"



This reverts commit fa247089.

The following sequence of commands causes a livelock - there will be
workqueue process looping and consuming 100% CPU:

dmsetup create --notable test
truncate -s 1MiB testdata
losetup /dev/loop0 testdata
dmsetup load test --table '0 2048 linear /dev/loop0 0'
dd if=/dev/zero of=/dev/dm-0 bs=16k count=1 conv=fdatasync

The livelock is caused by the commit fa247089. The commit claims that
it fixes a race condition, however, it is unknown what the actual race
condition is and what program is involved in the race condition.

When the inactive table is loaded, the nodes /dev/dm-0 and
/sys/block/dm-0 are created. /dev/dm-0 has zero size at this point. When
the device is suspended and resumed, the nodes /dev/mapper/test and
/dev/disk/* are created.

If some program opens a block device before it is created by dmsetup or
lvm, the program is buggy, so dm could just report an error as it used to
do before.

Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: fa247089 ("dm: requeue IO if mapping table not yet available")

parent 9c2010bc

drivers/md/dm-rq.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -496,8 +496,10 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,

		map = dm_get_live_table(md, &srcu_idx);
		if (unlikely(!map)) {
		DMERR_LIMIT("%s: mapping table unavailable, erroring io",
		dm_device_name(md));
		dm_put_live_table(md, srcu_idx);
		return BLK_STS_RESOURCE;
		return BLK_STS_IOERR;
		}
		ti = dm_table_find_target(map, 0);
		dm_put_live_table(md, srcu_idx);

drivers/md/dm.c

+8 −3

Original line number	Diff line number	Diff line
		@@ -2030,10 +2030,15 @@ static void dm_submit_bio(struct bio *bio)
		struct dm_table *map;

		map = dm_get_live_table(md, &srcu_idx);
		if (unlikely(!map)) {
		DMERR_LIMIT("%s: mapping table unavailable, erroring io",
		dm_device_name(md));
		bio_io_error(bio);
		goto out;
		}

		/* If suspended, or map not yet available, queue this IO for later */
		if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags)) \|\|
		unlikely(!map)) {
		/* If suspended, queue this IO for later */
		if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags))) {
		if (bio->bi_opf & REQ_NOWAIT)
		bio_wouldblock_error(bio);
		else if (bio->bi_opf & REQ_RAHEAD)