linux-kernel - Re: [PATCH 1/2] blk-mq: don't complete un-started request in timeout handler

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Date:   Wed, 15 Mar 2017 00:07:37 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "tom.leiming@...il.com" <tom.leiming@...il.com>,
        "axboe@...com" <axboe@...com>
CC:     "yizhan@...hat.com" <yizhan@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] blk-mq: don't complete un-started request in
 timeout handler

On Thu, 2017-03-09 at 21:02 +0800, Ming Lei wrote:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 159187a28d66..0aff380099d5 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -697,17 +697,8 @@ static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
>  {
>  	struct blk_mq_timeout_data *data = priv;
>  
> -	if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) {
> -		/*
> -		 * If a request wasn't started before the queue was
> -		 * marked dying, kill it here or it'll go unnoticed.
> -		 */
> -		if (unlikely(blk_queue_dying(rq->q))) {
> -			rq->errors = -EIO;
> -			blk_mq_end_request(rq, rq->errors);
> -		}
> +	if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
>  		return;
> -	}
>  
>  	if (time_after_eq(jiffies, rq->deadline)) {
>  		if (!blk_mark_rq_complete(rq))

Both the old and the new check look racy to me. The REQ_ATOM_STARTED bit can
be changed concurrently by blk_mq_start_request(), __blk_mq_finish_request()
or __blk_mq_requeue_request(). Another issue with this function is that the
request passed to this function can be reinitialized concurrently. Sorry but
I'm not sure what the best way is to address these issues.

Bart.