Re: [PATCH v2 2/4] block: add a read barrier in blk_queue_enter()

[Hannes Reinecke <hare@suse.de>]

On 03/24/2017 01:36 PM, Ming Lei wrote:
> Without the barrier, reading DEAD flag of .q_usage_counter
> and reading .mq_freeze_depth may be reordered, then the
> following wait_event_interruptible() may never return.
> 
> Signed-off-by: Ming Lei <tom.leiming@gmail.com>
> ---
>  block/blk-core.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index ad388d5e309a..44eed17319c0 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -669,6 +669,14 @@ int blk_queue_enter(struct request_queue *q, bool nowait)
>  		if (nowait)
>  			return -EBUSY;
>  
> +		/*
> +		 * read pair of barrier in blk_mq_freeze_queue_start(),
> +		 * we need to order reading DEAD flag of .q_usage_counter
> +		 * and reading .mq_freeze_depth, otherwise the following
> +		 * wait may never return if the two read are reordered.
> +		 */
> +		smp_rmb();
> +
>  		ret = wait_event_interruptible(q->mq_freeze_wq,
>  				!atomic_read(&q->mq_freeze_depth) ||
>  				blk_queue_dying(q));
> 
Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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