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

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Date:   Wed, 22 Mar 2017 12:35:18 -0400
From:   Jens Axboe <axboe@...com>
To:     Keith Busch <keith.busch@...el.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 03/22/2017 11:58 AM, Keith Busch wrote:
> 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.

Good, thanks for confirming, Keith. I queued up the patch for 4.11 this
morning.

-- 
Jens Axboe