linux-kernel - Re: [PATCH 3/5] sbitmap: fix improper use of smp_mb__before

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Message-ID: <20190510062727.GA4607@andrea>
Date:   Fri, 10 May 2019 08:27:27 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Omar Sandoval <osandov@...com>, linux-block@...r.kernel.org
Subject: Re: [PATCH 3/5] sbitmap: fix improper use of smp_mb__before_atomic()

Hi Ming,

On Fri, May 10, 2019 at 11:41:02AM +0800, Ming Lei wrote:
> On Mon, Apr 29, 2019 at 10:14:59PM +0200, Andrea Parri wrote:
> > This barrier only applies to the read-modify-write operations; in
> > particular, it does not apply to the atomic_set() primitive.
> > 
> > Replace the barrier with an smp_mb().
> > 
> > Fixes: 6c0ca7ae292ad ("sbitmap: fix wakeup hang after sbq resize")
> > Cc: stable@...r.kernel.org
> > Reported-by: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> > Reported-by: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> > Cc: Jens Axboe <axboe@...nel.dk>
> > Cc: Omar Sandoval <osandov@...com>
> > Cc: linux-block@...r.kernel.org
> > ---
> >  lib/sbitmap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index 155fe38756ecf..4a7fc4915dfc6 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -435,7 +435,7 @@ static void sbitmap_queue_update_wake_batch(struct sbitmap_queue *sbq,
> >  		 * to ensure that the batch size is updated before the wait
> >  		 * counts.
> >  		 */
> > -		smp_mb__before_atomic();
> > +		smp_mb();
> >  		for (i = 0; i < SBQ_WAIT_QUEUES; i++)
> >  			atomic_set(&sbq->ws[i].wait_cnt, 1);
> >  	}
> > -- 
> > 2.7.4
> > 
> 
> sbitmap_queue_update_wake_batch() won't be called in fast path, and
> the fix is correct too, so:
> 
> Reviewed-by: Ming Lei <ming.lei@...hat.com>

Thank you for the review(s),

  Andrea


> thanks,
> Ming