On Wed, 2017-03-15 at 16:22 -0400, Mikulas Patocka wrote:
> Hi
>
> This is backport of the upstram patch
> d67a5f4b5947aba4bfe9a80a2b86079c215ca755 for stable branches 3.16 and 4.1.
Queued up for 3.16, thanks.
Ben.
> Mikulas
>
>
> commit d67a5f4b5947aba4bfe9a80a2b86079c215ca755
> Author: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Date: Wed Feb 15 11:26:10 2017 -0500
>
> dm: flush queued bios when process blocks to avoid deadlock
>
> Commit df2cb6daa4 ("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: df2cb6daa4 ("block: Avoid deadlocks with bio allocation by stacking drivers")
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>
> ---
> drivers/md/dm.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 55 insertions(+)
>
> Index: linux-stable/drivers/md/dm.c
> ===================================================================
> --- linux-stable.orig/drivers/md/dm.c
> +++ linux-stable/drivers/md/dm.c
> @@ -1435,11 +1435,62 @@ void dm_accept_partial_bio(struct bio *b
> }
> 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, ¤t->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 mapped_device *md;
> + struct dm_offload o;
> struct bio *clone = &tio->clone;
> struct dm_target *ti = tio->ti;
>
> @@ -1452,7 +1503,11 @@ static void __map_bio(struct dm_target_i
> */
> 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 */
>
--
Ben Hutchings
compatible: Gracefully accepts erroneous data from any source
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