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

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Date:   Wed, 22 Mar 2017 11:58:17 -0400
From:   Keith Busch <keith.busch@...el.com>
To:     Jens Axboe <axboe@...com>
Cc:     Ming Lei <tom.leiming@...il.com>, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Yi Zhang <yizhan@...hat.com>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH] blk-mq: don't complete un-started request in timeout
 handler

On Tue, Mar 21, 2017 at 11:03:59PM -0400, Jens Axboe wrote:
> On 03/21/2017 10:14 PM, Ming Lei wrote:
> > When iterating busy requests in timeout handler,
> > if the STARTED flag of one request isn't set, that means
> > the request is being processed in block layer or driver, and
> > isn't submitted to hardware yet.
> > 
> > In current implementation of blk_mq_check_expired(),
> > if the request queue becomes dying, un-started requests are
> > handled as being completed/freed immediately. This way is
> > wrong, and can cause rq corruption or double allocation[1][2],
> > when doing I/O and removing&resetting NVMe device at the sametime.
> 
> I agree, completing it looks bogus. If the request is in a scheduler or
> on a software queue, this won't end well at all. Looks like it was
> introduced by this patch:
> 
> commit eb130dbfc40eabcd4e10797310bda6b9f6dd7e76
> Author: Keith Busch <keith.busch@...el.com>
> Date:   Thu Jan 8 08:59:53 2015 -0700
> 
>     blk-mq: End unstarted requests on a dying queue
> 
> Before that, we just ignored it. Keith?

The above was intended for a stopped hctx on a dying queue such that
there's nothing in flight to the driver. Nvme had been relying on this
to end unstarted requests so we may progress when a controller dies.

We've since obviated the need: we restart the hw queues to flush entered
requests to failure, so we don't need that brokenness.