Re: [PATCH RFC v3 2/3] sbitmap: fix invalid wakeup on the wrong waitqueue

[Yu Kuai]

Hi!

在 2022/07/11 22:26, Jan Kara 写道:
> On Sun 10-07-22 12:21:59, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > For example, 2 * wake_batch tags are put, while only wake_batch threads
> > are woken:
> >
> > __sbq_wake_up
> >   atomic_cmpxchg -> reset wait_cnt
> > 			__sbq_wake_up -> decrease wait_cnt
> > 			...
> > 			__sbq_wake_up -> wait_cnt is decreased to 0 again
> > 			 atomic_cmpxchg
> > 			 sbq_index_atomic_inc -> increase wake_index
> > 			 wake_up_nr -> wake up and waitqueue might be empty
> >   sbq_index_atomic_inc -> increase again, one waitqueue is skipped
> >   wake_up_nr -> invalid wake up because old wakequeue might be empty
> >
> > To fix the problem, increasing 'wake_index' before resetting 'wait_cnt'.
> >
> > Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library")
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> This patch and the following one look sane to me but please merge them to a
> single patch. They fix the same race of two concurrent wakers just with a
> slightly different timing so there isn't a point in having two patches for
> this (in particular changes in this patch are difficult to reason about
> when we know the result is still buggy).

Ok, I'll merge them.

Thanks,
Kuai
> 								Honza
>
> > ---
> >   lib/sbitmap.c | 45 +++++++++++++++++++++++----------------------
> >   1 file changed, 23 insertions(+), 22 deletions(-)
> >
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index b46fce1beb3a..57095dd88a33 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -616,32 +616,33 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> >   		return false;
> >   
> >   	wait_cnt = atomic_dec_return(&ws->wait_cnt);
> > -	if (wait_cnt <= 0) {
> > -		int ret;
> > +	if (wait_cnt > 0)
> > +		return false;
> >   
> > -		wake_batch = READ_ONCE(sbq->wake_batch);
> > +	/*
> > +	 * For concurrent callers of this, callers should call this function
> > +	 * again to wakeup a new batch on a different 'ws'.
> > +	 */
> > +	if (wait_cnt < 0)
> > +		return true;
> >   
> > -		/*
> > -		 * Pairs with the memory barrier in sbitmap_queue_resize() to
> > -		 * ensure that we see the batch size update before the wait
> > -		 * count is reset.
> > -		 */
> > -		smp_mb__before_atomic();
> > +	wake_batch = READ_ONCE(sbq->wake_batch);
> >   
> > -		/*
> > -		 * For concurrent callers of this, the one that failed the
> > -		 * atomic_cmpxhcg() race should call this function again
> > -		 * to wakeup a new batch on a different 'ws'.
> > -		 */
> > -		ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
> > -		if (ret == wait_cnt) {
> > -			sbq_index_atomic_inc(&sbq->wake_index);
> > -			wake_up_nr(&ws->wait, wake_batch);
> > -			return false;
> > -		}
> > +	/*
> > +	 * Pairs with the memory barrier in sbitmap_queue_resize() to
> > +	 * ensure that we see the batch size update before the wait
> > +	 * count is reset.
> > +	 */
> > +	smp_mb__before_atomic();
> >   
> > -		return true;
> > -	}
> > +	/*
> > +	 * Increase wake_index before updating wait_cnt, otherwise concurrent
> > +	 * callers can see valid wait_cnt in old waitqueue, which can cause
> > +	 * invalid wakeup on the old waitqueue.
> > +	 */
> > +	sbq_index_atomic_inc(&sbq->wake_index);
> > +	atomic_set(&ws->wait_cnt, wake_batch);
> > +	wake_up_nr(&ws->wait, wake_batch);
> >   
> >   	return false;
> >   }
> > --
> > 2.31.1