linux-kernel - Re: locking/atomic: Introduce atomic_try

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Message-ID: <CALCETrWAcahnXNEXZjyBRFmcyCLER0_0tpQG3Ckv2qGJU5TYWQ@mail.gmail.com>
Date:   Fri, 24 Mar 2017 13:27:49 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()

On Fri, Mar 24, 2017 at 1:22 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Mar 24, 2017 at 11:45:46AM -0700, Andy Lutomirski wrote:
>> After playing with it a bit, I found some of the problem: you're
>> passing val into EXCEPTION_VALUE, which keeps it live.  If I get rid
>> of that, the generated code is great.
>
> Right, so I needed that because I land on ud2 through 2 different paths:
>
>  - newly saturated
>  - use-after-free
>
> And the exception handler can figure out which of the two by looking at
> the variable, but then of course, it needs to be life.
>
> For the full horror of how to do this, look here:
>
>   http://paste.debian.net/924190/
>
> But I didn't just show you that, so you can't blame me for any damage
> that might've done you.

Wow, that's horrible.  Could this not be done by looking at flags
instead of regs?

For that matter, you're effectively comparing to -1 and 0.  I'm not
really sure it would be faster, but you could plausibly add one then
subtract one again and get the full picture just from flags and a
single comparison?