[dm-devel] [PATCH 3/3] dm: mark dm queue as blocking if any underlying is blocking

[Ming Lei]

On Thu, Jan 06, 2022 at 10:40:51AM -0500, Mike Snitzer wrote:
> On Tue, Dec 21 2021 at  9:14P -0500,
> Ming Lei <ming.lei at redhat.com> wrote:
> 
> > dm request based driver doesn't set BLK_MQ_F_BLOCKING, so dm_queue_rq()
> > is supposed to not sleep.
> > 
> > However, blk_insert_cloned_request() is used by dm_queue_rq() for
> > queuing underlying request, but the underlying queue may be marked as
> > BLK_MQ_F_BLOCKING, so blk_insert_cloned_request() may become to block
> > current context, then rcu warning is triggered.
> > 
> > Fixes the issue by marking dm request based queue as BLK_MQ_F_BLOCKING
> > if any underlying queue is marked as BLK_MQ_F_BLOCKING, meantime we
> > need to allocate srcu beforehand.
> > 
> > Signed-off-by: Ming Lei <ming.lei at redhat.com>
> > ---
> >  drivers/md/dm-rq.c    |  5 ++++-
> >  drivers/md/dm-rq.h    |  3 ++-
> >  drivers/md/dm-table.c | 14 ++++++++++++++
> >  drivers/md/dm.c       |  5 +++--
> >  drivers/md/dm.h       |  1 +
> >  5 files changed, 24 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
> > index 579ab6183d4d..2297d37c62a9 100644
> > --- a/drivers/md/dm-rq.c
> > +++ b/drivers/md/dm-rq.c
> > @@ -535,7 +535,8 @@ static const struct blk_mq_ops dm_mq_ops = {
> >  	.init_request = dm_mq_init_request,
> >  };
> >  
> > -int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t)
> > +int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t,
> > +			     bool blocking)
> >  {
> >  	struct dm_target *immutable_tgt;
> >  	int err;
> > @@ -550,6 +551,8 @@ int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t)
> >  	md->tag_set->flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_STACKING;
> >  	md->tag_set->nr_hw_queues = dm_get_blk_mq_nr_hw_queues();
> >  	md->tag_set->driver_data = md;
> > +	if (blocking)
> > +		md->tag_set->flags |= BLK_MQ_F_BLOCKING;
> >  
> >  	md->tag_set->cmd_size = sizeof(struct dm_rq_target_io);
> >  	immutable_tgt = dm_table_get_immutable_target(t);
> 
> As you can see, dm_table_get_immutable_target(t) is called here ^
> 
> Rather than pass 'blocking' in, please just call dm_table_has_blocking_dev(t);
> 
> But not a big deal, I can clean that up once this gets committed...
> 
> > diff --git a/drivers/md/dm-rq.h b/drivers/md/dm-rq.h
> > index 1eea0da641db..5f3729f277d7 100644
> > --- a/drivers/md/dm-rq.h
> > +++ b/drivers/md/dm-rq.h
> > @@ -30,7 +30,8 @@ struct dm_rq_clone_bio_info {
> >  	struct bio clone;
> >  };
> >  
> > -int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t);
> > +int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t,
> > +			     bool blocking);
> >  void dm_mq_cleanup_mapped_device(struct mapped_device *md);
> >  
> >  void dm_start_queue(struct request_queue *q);
> > diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> > index aa173f5bdc3d..e4bdd4f757a3 100644
> > --- a/drivers/md/dm-table.c
> > +++ b/drivers/md/dm-table.c
> > @@ -1875,6 +1875,20 @@ static bool dm_table_supports_write_zeroes(struct dm_table *t)
> >  	return true;
> >  }
> >  
> > +/* If the device can block inside ->queue_rq */
> > +static int device_is_io_blocking(struct dm_target *ti, struct dm_dev *dev,
> > +			      sector_t start, sector_t len, void *data)
> > +{
> > +	struct request_queue *q = bdev_get_queue(dev->bdev);
> > +
> > +	return blk_queue_blocking(q);
> > +}
> > +
> > +bool dm_table_has_blocking_dev(struct dm_table *t)
> > +{
> > +	return dm_table_any_dev_attr(t, device_is_io_blocking, NULL);
> > +}
> > +
> >  static int device_not_nowait_capable(struct dm_target *ti, struct dm_dev *dev,
> >  				     sector_t start, sector_t len, void *data)
> >  {
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index 280918cdcabd..2f72877752dd 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -1761,7 +1761,7 @@ static struct mapped_device *alloc_dev(int minor)
> >  	 * established. If request-based table is loaded: blk-mq will
> >  	 * override accordingly.
> >  	 */
> > -	md->disk = blk_alloc_disk(md->numa_node_id);
> > +	md->disk = blk_alloc_disk_srcu(md->numa_node_id);
> >  	if (!md->disk)
> >  		goto bad;
> >  	md->queue = md->disk->queue;
> > @@ -2046,7 +2046,8 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t)
> >  	switch (type) {
> >  	case DM_TYPE_REQUEST_BASED:
> >  		md->disk->fops = &dm_rq_blk_dops;
> > -		r = dm_mq_init_request_queue(md, t);
> > +		r = dm_mq_init_request_queue(md, t,
> > +				dm_table_has_blocking_dev(t));
> >  		if (r) {
> >  			DMERR("Cannot initialize queue for request-based dm mapped device");
> >  			return r;
> > diff --git a/drivers/md/dm.h b/drivers/md/dm.h
> > index 742d9c80efe1..f7f92b272cce 100644
> > --- a/drivers/md/dm.h
> > +++ b/drivers/md/dm.h
> > @@ -60,6 +60,7 @@ int dm_calculate_queue_limits(struct dm_table *table,
> >  			      struct queue_limits *limits);
> >  int dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
> >  			      struct queue_limits *limits);
> > +bool dm_table_has_blocking_dev(struct dm_table *t);
> >  struct list_head *dm_table_get_devices(struct dm_table *t);
> >  void dm_table_presuspend_targets(struct dm_table *t);
> >  void dm_table_presuspend_undo_targets(struct dm_table *t);
> > -- 
> > 2.31.1
> > 
> 
> Reviewed-by: Mike Snitzer <snitzer at redhat.com>

Thanks!

> 
> Late, given holidays we know why, but this patchset is needed for 5.17
> (maybe with added: 'Fixes: 704b914f15fb7 "blk-mq: move srcu from
> blk_mq_hw_ctx to request_queue"' to this 3rd patch?)

It is one long-term issue, not related with commit 704b914f15fb7. The
problem is that rcu read lock is held by blk-mq when running dm_queue_rq()
which calls underlying blocking queue's ->queue_rq() which may sleep
somewhere.


Thanks,
Ming