Re: locking/atomic: Introduce atomic_try_cmpxchg()

From: Andy Lutomirski <luto@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Brian Gerst <brgerst@gmail.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()
Date: Fri, 24 Mar 2017 13:27:49 -0700	[thread overview]
Message-ID: <CALCETrWAcahnXNEXZjyBRFmcyCLER0_0tpQG3Ckv2qGJU5TYWQ@mail.gmail.com> (raw)
In-Reply-To: <20170324202242.GA5680@worktop>

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