linux-kernel - Re: [PATCH] x86/split_lock: Restore warn mode (and add a new one) to avoid userspace regression

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Date:   Sun, 30 Oct 2022 16:13:15 +0000
From:   Lukas Straub <lukasstraub2@....de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "kernel-dev@...lia.com" <kernel-dev@...lia.com>,
        "kernel@...ccoli.net" <kernel@...ccoli.net>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Joshua Ashton <joshua@...ggi.es>,
        Paul Gofman <pgofman@...eweavers.com>,
        Pavel Machek <pavel@...x.de>,
        Pierre-Loup Griffais <pgriffais@...vesoftware.com>,
        "Melissa Wen" <mwen@...lia.com>
Subject: Re: [PATCH] x86/split_lock: Restore warn mode (and add a new one)
 to avoid userspace regression

On Thu, 29 Sep 2022 15:37:55 +0000
"Luck, Tony" <tony.luck@...el.com> wrote:

> >> I have a revert removing the misery ready and tested, let me know if I
> >> should submit it.  
> >
> > I'm a bit of a late arrival to the split lock party, so I'm a bit
> > hesitant to merge any changes immediately.
> >
> > How about we give it a few weeks and see if the current behavior impacts
> > anyone else?  Maybe the best route will be more clear then.  
> 
> Applying "misery" to the processes that are executing split-lock flows saves
> the rest of the system from a different level of misery (for the duration of the
> split lock other logical CPUs and I/O devices have access to memory blocked).
> 
> So the "misery" serves a very useful purpose on multi-user systems.

Hello Everyone,
How about the following: The kernel traps the split-lock, but instead
of punishing the process artificially it emulates it in a different way
that won't harm the system as a whole. Of course this still will be
slower than a non-split-lock but surely won't take 10ms.

For example, you could emulate the instruction without atomic semantics,
but protected under a single global mutex for all split-lock operations
instead. This is how atomics are done on on alpha AFAIK, which doesn't
have atomic instructions.

This is not as simple as the current solution. But I see the current
solution more like a quick and dirty workaround for this security/DoS
issue, until a proper solution (like my proposal) is implemented.

Regards,
Lukas Straub

> Maybe the decision of which mode to use could be dynamic based on
> number of online CPUs? Laptops/desktops with low counts (<50???)
> could just "warn", while servers could default to the "seq" mode.
> 
> Or perhaps there is some other heuristic to distinguish single-user
> systems where the split-locks are not causing pain to other users?
> 
> -Tony



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