Commit d67a5f4b authored Feb 15, 2017 by Mikulas Patocka Committed by Mike Snitzer Feb 17, 2017

dm: flush queued bios when process blocks to avoid deadlock

Commit df2cb6da ("block: Avoid deadlocks with bio allocation by
stacking drivers") created a workqueue for every bio set and code
in bio_alloc_bioset() that tries to resolve some low-memory deadlocks
by redirecting bios queued on current->bio_list to the workqueue if the
system is low on memory. However other deadlocks (see below **) may
happen, without any low memory condition, because generic_make_request
is queuing bios to current->bio_list (rather than submitting them).

** the related dm-snapshot deadlock is detailed here:
https://www.redhat.com/archives/dm-devel/2016-July/msg00065.html

Fix this deadlock by redirecting any bios on current->bio_list to the
bio_set's rescue workqueue on every schedule() call. Consequently,
when the process blocks on a mutex, the bios queued on
current->bio_list are dispatched to independent workqueus and they can
complete without waiting for the mutex to be available.

The structure blk_plug contains an entry cb_list and this list can contain
arbitrary callback functions that are called when the process blocks.
To implement this fix DM (ab)uses the onstack plug's cb_list interface
to get its flush_current_bio_list() called at schedule() time.

This fixes the snapshot deadlock - if the map method blocks,
flush_current_bio_list() will be called and it redirects bios waiting
on current->bio_list to appropriate workqueues.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1267650

Depends-on: df2cb6da ("block: Avoid deadlocks with bio allocation by stacking drivers")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

parent 37a098e9

drivers/md/dm.c

+55 −0

Original line number	Diff line number	Diff line
		@@ -972,10 +972,61 @@ void dm_accept_partial_bio(struct bio *bio, unsigned n_sectors)
		}
		EXPORT_SYMBOL_GPL(dm_accept_partial_bio);

		/*
		* Flush current->bio_list when the target map method blocks.
		* This fixes deadlocks in snapshot and possibly in other targets.
		*/
		struct dm_offload {
		struct blk_plug plug;
		struct blk_plug_cb cb;
		};

		static void flush_current_bio_list(struct blk_plug_cb *cb, bool from_schedule)
		{
		struct dm_offload *o = container_of(cb, struct dm_offload, cb);
		struct bio_list list;
		struct bio *bio;

		INIT_LIST_HEAD(&o->cb.list);

		if (unlikely(!current->bio_list))
		return;

		list = *current->bio_list;
		bio_list_init(current->bio_list);

		while ((bio = bio_list_pop(&list))) {
		struct bio_set *bs = bio->bi_pool;
		if (unlikely(!bs) \|\| bs == fs_bio_set) {
		bio_list_add(current->bio_list, bio);
		continue;
		}

		spin_lock(&bs->rescue_lock);
		bio_list_add(&bs->rescue_list, bio);
		queue_work(bs->rescue_workqueue, &bs->rescue_work);
		spin_unlock(&bs->rescue_lock);
		}
		}

		static void dm_offload_start(struct dm_offload *o)
		{
		blk_start_plug(&o->plug);
		o->cb.callback = flush_current_bio_list;
		list_add(&o->cb.list, &current->plug->cb_list);
		}

		static void dm_offload_end(struct dm_offload *o)
		{
		list_del(&o->cb.list);
		blk_finish_plug(&o->plug);
		}

		static void __map_bio(struct dm_target_io *tio)
		{
		int r;
		sector_t sector;
		struct dm_offload o;
		struct bio *clone = &tio->clone;
		struct dm_target *ti = tio->ti;

		@@ -988,7 +1039,11 @@ static void __map_bio(struct dm_target_io *tio)
		*/
		atomic_inc(&tio->io->io_count);
		sector = clone->bi_iter.bi_sector;

		dm_offload_start(&o);
		r = ti->type->map(ti, clone);
		dm_offload_end(&o);

		if (r == DM_MAPIO_REMAPPED) {
		/* the bio has been remapped so dispatch it */

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