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

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Date:   Fri, 24 Mar 2017 22:07:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.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 01:27:49PM -0700, Andy Lutomirski wrote:
> 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?

Well, the EXCEPTION_HANDLER() thing is something ARM/ARM64 could also
implement.