linux-kernel - Re: [PATCH 2/2] blk-mq: Fix request handover from timeout path to normal execution

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Message-ID: <6d6985b8d95317aa5572809f1b987c45e932c5b2.camel@wdc.com>
Date:   Mon, 2 Apr 2018 22:09:18 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "tj@...nel.org" <tj@...nel.org>
CC:     "kernel-team@...com" <kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Subject: Re: [PATCH 2/2] blk-mq: Fix request handover from timeout path to
 normal execution

On Mon, 2018-04-02 at 15:01 -0700, tj@...nel.org wrote:
> On Mon, Apr 02, 2018 at 09:56:41PM +0000, Bart Van Assche wrote:
> > This patch increases the time during which .aborted_gstate == .gstate if the
> > timeout is reset. Does that increase the chance that a completion will be missed
> > if the request timeout is reset?
> 
> It sure does, but we're comparing an outright kernel bug vs. an
> inherently opportunistic mechanism being a bit more lossy.  I think
> the answer is pretty clear.

Hello Tejun,

Please elaborate what your long-term goal is for the blk-mq timeout handler.
The legacy block layer suspends request state changes while a timeout is
being processed by holding the request queue lock while requests are being
processed, while processing a timeout and while calling q->rq_timed_out_fn(rq).
Do you think it is possible to make the blk-mq core suspend request processing
while processing a timeout without introducing locking in
blk_mq_complete_request()? If you do not plan to add locking in
blk_mq_complete_request(), do you think it is possible to fix all the races we
discussed in previous e-mails?

Thanks,

Bart.