Re: [PATCH] block: don't dereference request after flush insertion

[Ming Lei <ming.lei@redhat.com>]

On Sat, Oct 16, 2021 at 07:35:39PM -0600, Jens Axboe wrote:
> We could have a race here, where the request gets freed before we call
> into blk_mq_run_hw_queue(). If this happens, we cannot rely on the state
> of the request.
> 
> Grab the hardware context before inserting the flush.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 2197cfbf081f..22b30a89bf3a 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2468,9 +2468,10 @@ void blk_mq_submit_bio(struct bio *bio)
>  	}
>  
>  	if (unlikely(is_flush_fua)) {
> +		struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
>  		/* Bypass scheduler for flush requests */
>  		blk_insert_flush(rq);
> -		blk_mq_run_hw_queue(rq->mq_hctx, true);
> +		blk_mq_run_hw_queue(hctx, true);
>  	} else if (plug && (q->nr_hw_queues == 1 ||
>  		   blk_mq_is_shared_tags(rq->mq_hctx->flags) ||
>  		   q->mq_ops->commit_rqs || !blk_queue_nonrot(q))) {

From report in [1], no device close & queue release is involved, and
request freeing could be much easier to trigger than queue release,
so looks fine:

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Fixes: f328476e373a ("blk-mq: cleanup blk_mq_submit_bio")



[1] https://lore.kernel.org/linux-block/23531d29-9d96-6744-bab9-797e65379037@kernel.dk/T/#t


thanks,
Ming