Re: locking/atomic: Introduce atomic_try_cmpxchg()

[Andy Lutomirski]

On Fri, Mar 24, 2017 at 1:22 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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?