linux-kernel - Re: [PATCH 6.0.y / 6.1.y] x86/split_lock: Add sysctl to control the misery mode

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Date:   Wed, 29 Mar 2023 22:43:38 +0500
From:   stsp <stsp2@...dex.ru>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, Greg KH <greg@...ah.com>,
        stable@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, corbet@....net,
        kernel-dev@...lia.com, kernel@...ccoli.net, fenghua.yu@...el.com,
        joshua@...ggi.es, pgofman@...eweavers.com, pavel@...x.de,
        pgriffais@...vesoftware.com, zfigura@...eweavers.com,
        cristian.ciocaltea@...labora.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andre Almeida <andrealmeid@...lia.com>
Subject: Re: [PATCH 6.0.y / 6.1.y] x86/split_lock: Add sysctl to control the
 misery mode

Hi Sean, thanks for a head-up!

29.03.2023 22:14, Sean Christopherson пишет:
> Resurrecting this thread with a concrete use case.
> dosemu2, which uses KVM to accelerate DOS emulation (stating the obvious), ran
> into a problem where the hardcoded (prior to this patch) behavior will effectively
> hang userspace if the 10ms sleep is interrupted, e.g. by a periodic SIGALRM[*].

Yes, and we also created a ready-to-use
host test-case with no dosemu2 involved.

> Alternatively, we could try to figure out a way to ensure forward progress without
> letting userpace run an end-around on the enforced misery, but backporting this
> patch to stable kernels seems easier.
>
> Stas, do you happen to know what the oldest stable kernel your users use, i.e.
> how far back this backport would need to go?
It seems, the "broken" code was added by
the patch b041b525dab95 which is "described"
as v5.18-rc4-1-gb041b525dab9.
So I guess the answer would be "down to 5.18".

Of course I don't believe the patch you
mention, is a real solution. Its good for
-stable, but something else needs to be
developed in the future.