linux-kernel - Re: [PATCH 00/18] x86: Ticket lock + cmpxchg cleanup

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Message-ID: <4E5589A4.8000606@goop.org>
Date:	Wed, 24 Aug 2011 16:30:44 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	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 00/18] x86: Ticket lock + cmpxchg cleanup

On 08/24/2011 04:21 PM, Linus Torvalds wrote:
> On Wed, Aug 24, 2011 at 4:09 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> However, the reason I disagree with it is that I don't think that it's
>> any prettier at all to have the two barriers than it is to just have
>> the asm.
> .. and btw, we probably do need *both* barriers. We definitely need
> the one before. The one after is a bit less obvious, since it is
> technically legal for code to move into the locked region. However,
> it's not necessarily a *good* idea for code to move into the locked
> region, so the two barriers are likely the RightThing(tm).

Originally I left the second barrier off for that reason, but I got
mysterious lockups.  The second barrier fixed them, so I never got
around to do a full root-cause analysis.

I still think the C version is more straightforward given that the asm
version is confused with the details of the ticket sizes, etc.  But,
shrug, its a pretty minor detail.

The OOSTORE stuff is a complete red herring; I bet its been *years*
since someone specifically compiled a kernel with OOSTORE SMP support
because they actually wanted to use it.

    J
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