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

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Message-ID: <20180402221657.GL388343@devbig577.frc2.facebook.com>
Date:   Mon, 2 Apr 2018 15:16:57 -0700
From:   "tj@...nel.org" <tj@...nel.org>
To:     Bart Van Assche <Bart.VanAssche@....com>
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

Hello,

On Mon, Apr 02, 2018 at 10:09:18PM +0000, Bart Van Assche wrote:
> Please elaborate what your long-term goal is for the blk-mq timeout handler.

Hmm... I don't really have any plans beyond what's been posted.

> 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?

I don't know of multiple race conditions.  What am I missing?  AFAIK,
there's one non-critical race condition which has always been there.
We have a larger race window for that case but don't yet know whether
that's problematic or not.  If that actually is problematic, we can
figure out a way to solve that but such effort / added complexity
doesn't seem justified yet.  No?

Thanks.

-- 
tejun