linux-kernel - Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug

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Message-ID: <48A47B83.3090408@zytor.com>
Date:	Thu, 14 Aug 2008 11:37:55 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <srostedt@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with	preemptible
 kernel and CPU hotplug

Jeremy Fitzhardinge wrote:
> 
> Ah, OK.  I'd thought we started unlocked, but given that I've just been 
> disassembling the kernel and looking at the lock prefixes, that's a bit 
> of a braino on my part.
> 
> BTW, using the ds prefix allows us to undo the hack of dealing with 
> locked instructions with exception handlers.  There was a bug where if 
> we do lock->nop, then the address of a faulting instruction changes, so 
> we need exception records for both the locked and unlocked forms.  Using 
> ds means the instruction size doesn't change, so we only need one 
> exception record.  I don't remember off hand where that happens.
> 

Using %ds: rather than nop really seems to solve a whole lot of 
problems, and might even be faster to boot.  It really sounds like a 
no-brainer.

	-hpa
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