linux-kernel - Re: [PATCHSET] blk-mq: reimplement timeout handling

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Date:   Mon, 11 Dec 2017 10:27:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org, oleg@...hat.com,
        kernel-team@...com, osandov@...com
Subject: Re: [PATCHSET] blk-mq: reimplement timeout handling

On Sat, Dec 09, 2017 at 11:25:19AM -0800, Tejun Heo wrote:
> Currently, blk-mq timeout path synchronizes against the usual
> issue/completion path using a complex scheme involving atomic
> bitflags, REQ_ATOM_*, memory barriers and subtle memory coherence
> rules.  Unfortunatley, it contains quite a few holes.
> 
> It's pretty easy to make blk_mq_check_expired() terminate a later
> instance of a request.  If we induce 5 sec delay before
> time_after_eq() test in blk_mq_check_expired(), shorten the timeout to
> 2s, and issue back-to-back large IOs, blk-mq starts timing out
> requests spuriously pretty quickly.  Nothing actually timed out.  It
> just made the call on a recycle instance of a request and then
> terminated a later instance long after the original instance finished.
> The scenario isn't theoretical either.
> 
> This patchset replaces the broken synchronization mechanism with a RCU
> and generation number based one.  Please read the patch description of
> the second path for more details.
> 
> Oleg, Peter, I'd really appreciate if you guys can go over the
> reported breakages and the new implementation.

Great, yes that code seemed very suspicious when I looked at it; thanks
for making it go away. I'll try and find a spot to stare at the patches.