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Date:   Fri, 10 May 2019 11:41:02 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org, stable@...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()

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>

thanks,
Ming

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