linux-kernel - Re: [PATCH v9 2/2] blk-mq: Rework blk-mq timeout handling again

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Date:   Wed, 16 May 2018 16:47:54 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "hch@....de" <hch@....de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "israelr@...lanox.com" <israelr@...lanox.com>,
        "sagi@...mberg.me" <sagi@...mberg.me>,
        "sebott@...ux.ibm.com" <sebott@...ux.ibm.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "maxg@...lanox.com" <maxg@...lanox.com>,
        "tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH v9 2/2] blk-mq: Rework blk-mq timeout handling again

On Wed, 2018-05-16 at 18:24 +0200, hch@....de wrote:
> On Wed, May 16, 2018 at 04:17:42PM +0000, Bart Van Assche wrote:
> > There is another reason the deadline is included in the atomic operation,
> > namely to handle races between the BLK_EH_RESET_TIMER case in blk_mq_rq_timed_out()
> > and blk_mq_complete_request(). I don't think that race is addressed properly by
> > your patch. I will see what I can do to address that race without using 64-bit
> > atomic operations.
> 
> I might be missing something here, so please help me understand
> what is missing.
> 
> If we restart the timer in blk_mq_rq_timed_out we also bump the
> generation at the same time as we reset the deadline in your old
> patch.  With this patch we only bump the generation, but that should
> be enough to address the rest, or not?

Hello Christoph,

I think your patch changes the order of changing the request state and
calling mod_timer(). In my patch the request state and the deadline are
updated first and mod_timer() is called afterwards. I think your patch
changes the order of these operations into the following:
(1) __blk_mq_start_request() sets the request deadline.
(2) __blk_mq_start_request() calls __blk_add_timer() which in turn calls
    mod_timer().
(3) __blk_mq_start_request() changes the request state into MQ_RQ_IN_FLIGHT.

In the unlikely event of a significant delay between (2) and (3) it can
happen that the timer fires and examines and ignores the request because
its state differs from MQ_RQ_IN_FLIGHT. If the request for which this
happened times out its timeout will only be handled the next time
blk_mq_timeout_work() is called. Is this the behavior you intended?

Thanks,

Bart.