linux-kernel - Re: [PATCH 3/8] blk-mq: replace timeout synchronization with a RCU and generation based scheme

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Date:   Mon, 8 Jan 2018 21:06:55 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "jbacik@...com" <jbacik@...com>, "tj@...nel.org" <tj@...nel.org>,
        "jack@...e.cz" <jack@...e.cz>, "clm@...com" <clm@...com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
CC:     "kernel-team@...com" <kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "hch@....de" <hch@....de>
Subject: Re: [PATCH 3/8] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

On Mon, 2018-01-08 at 11:15 -0800, Tejun Heo wrote:
> +static void blk_mq_rq_update_aborted_gstate(struct request *rq, u64 gstate)
> +{
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	u64_stats_update_begin(&rq->aborted_gstate_sync);
> +	rq->aborted_gstate = gstate;
> +	u64_stats_update_end(&rq->aborted_gstate_sync);
> +	local_irq_restore(flags);
> +}

Please add a comment that explains the purpose of local_irq_save() and
local_irq_restore(). Please also explain why you chose to disable interrupts
instead of disabling preemption. I think that disabling preemption should be
sufficient since this is the only code that updates rq->aborted_gstate and
since this function is always called from thread context.

> @@ -801,6 +840,12 @@ void blk_mq_rq_timed_out(struct request *req, bool reserved)
>  		__blk_mq_complete_request(req);
>  		break;
>  	case BLK_EH_RESET_TIMER:
> +		/*
> +		 * As nothing prevents from completion happening while
> +		 * ->aborted_gstate is set, this may lead to ignored
> +		 * completions and further spurious timeouts.
> +		 */
> +		blk_mq_rq_update_aborted_gstate(req, 0);
>  		blk_add_timer(req);
>  		blk_clear_rq_complete(req);
>  		break;

Is the race that the comment refers to addressed by one of the later patches?

Thanks,

Bart.