Re: [PATCH] blk-mq: fix IO hang from sbitmap wakeup race

[Jan Kara <jack@suse.cz>]

On Fri 12-01-24 17:27:48, Kemeng Shi wrote:
> 
> 
> on 1/11/2024 11:54 PM, Ming Lei wrote:
> > In blk_mq_mark_tag_wait(), __add_wait_queue() may be re-ordered
> > with the following blk_mq_get_driver_tag() in case of getting driver
> > tag failure.
> > 
> > Then in __sbitmap_queue_wake_up(), waitqueue_active() may not observe
> > the added waiter in blk_mq_mark_tag_wait() and wake up nothing, meantime
> > blk_mq_mark_tag_wait() can't get driver tag successfully.
> > 
> > This issue can be reproduced by running the following test in loop, and
> > fio hang can be observed in < 30min when running it on my test VM
> > in laptop.
> > 
> > 	modprobe -r scsi_debug
> > 	modprobe scsi_debug delay=0 dev_size_mb=4096 max_queue=1 host_max_queue=1 submit_queues=4
> > 	dev=`ls -d /sys/bus/pseudo/drivers/scsi_debug/adapter*/host*/target*/*/block/* | head -1 | xargs basename`
> > 	fio --filename=/dev/"$dev" --direct=1 --rw=randrw --bs=4k --iodepth=1 \
> >        		--runtime=100 --numjobs=40 --time_based --name=test \
> >         	--ioengine=libaio
> > 
> > Fix the issue by adding one explicit barrier in blk_mq_mark_tag_wait(), which
> > is just fine in case of running out of tag.
> > 
> > Apply the same pattern in blk_mq_get_tag() which should have same risk.
> > 
> > Reported-by: Changhui Zhong <czhong@redhat.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> > BTW, Changhui is planning to upstream the test case to blktests.
> > 
> >  block/blk-mq-tag.c | 19 +++++++++++++++++++
> >  block/blk-mq.c     | 16 ++++++++++++++++
> >  2 files changed, 35 insertions(+)
> > 
> > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> > index cc57e2dd9a0b..29f77cae8eb2 100644
> > --- a/block/blk-mq-tag.c
> > +++ b/block/blk-mq-tag.c
> > @@ -179,6 +179,25 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
> >  
> >  		sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
> >  
> > +		/*
> > +		 * Add one explicit barrier since __blk_mq_get_tag() may not
> > +		 * imply barrier in case of failure.
> > +		 *
> > +		 * Order adding us to wait queue and the following allocating
> > +		 * tag in  __blk_mq_get_tag().
> > +		 *
> > +		 * The pair is the one implied in sbitmap_queue_wake_up()
> > +		 * which orders clearing sbitmap tag bits and
> > +		 * waitqueue_active() in __sbitmap_queue_wake_up(), since
> > +		 * waitqueue_active() is lockless
> > +		 *
> > +		 * Otherwise, re-order of adding wait queue and getting tag
> > +		 * may cause __sbitmap_queue_wake_up() to wake up nothing
> > +		 * because the waitqueue_active() may not observe us in wait
> > +		 * queue.
> > +		 */
> > +		smp_mb();
> > +
> Hi Ming, thanks for the fix. I'm not sure if we should explicitly imply
> a memory barrier here as prepare_to_wait variants normally imply a general
> memory barrier (see section "SLEEP AND WAKE-UP FUNCTIONS " in [1]).
> Wish this helps!

Indeed, good spotting with the ordering bug Ming! I agree with Kemeng
though that set_current_state() called from sbitmap_prepare_to_wait() is
guaranteed to contain a memory barrier and thus reads from
__blk_mq_get_tag() are guaranteed to be ordered properly wrt addition into
the waitqueue.

So only blk_mq_mark_tag_wait() is vulnerable to the problem you have
spotted AFAICT.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR