linux-kernel - Re: [PATCH 13/15] x86: add cmpxchg

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4E5527C2.8000801@goop.org>
Date:	Wed, 24 Aug 2011 09:33:06 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Christoph Lameter <cl@...ux.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 13/15] x86: add cmpxchg_flag() variant

On 08/24/2011 06:53 AM, Christoph Lameter wrote:
> On Tue, 23 Aug 2011, Jeremy Fitzhardinge wrote:
>
>> However, having prototyped it, I dunno, it doesn't really seem like much
>> of a win for all the extra code it adds.  I just can't get too excited
>> about an extra test instruction adjacent to a monster like a locked
>> cmpxchg.  The jump variant avoids the test, but gcc still generates some
>> pretty bogus stuff:
> There are also unlocked cmpxchges in use.

I only looked in arch/x86, but I didn't find any that were
straightforward candidates for cmpxchg_flag.

>  And if the cmpxchg is a 16 byte
> cmpxchg (cmpxchg_double) then the comparison is getting more expensive.

We're talking about the difference between cmpxchg_flag()  - which does
a sete based on the flags set up cmpxchg - and a variant based on "asm
goto" which could, in principle, avoid the need for sete by allowing a
control flow statment to directly use the asm goto's conditional jump. 
The performance of both is invariant wrt the cmpxchg argument size.

    J
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/