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

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Date:   Wed, 16 May 2018 18:06:16 +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>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "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 19:31 +0200, hch@....de wrote:
> On Wed, May 16, 2018 at 04:47:54PM +0000, Bart Van Assche wrote:
> > 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?
> 
> We can move the timer manipulation after the change easily I think.
> It would make sense to add comments explaining the ordering.

Hello Christoph,

I'm afraid that could lead to mod_timer() being called in another order than
intended. If e.g. the code that handles BLK_EH_RESET_TIMER changes the request
state first to in-flight and next calls mod_timer() then it can happen that
another context completes and restarts the request, resulting in a concurrent
mod_timer() call. I'm not sure reordering of the mod_timer() calls would result
in correct behavior.

Bart.