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

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Message-ID: <20080814185338.GB7896@Krystal>
Date:	Thu, 14 Aug 2008 14:53:38 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

* H. Peter Anvin (hpa@...or.com) wrote:
> 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

So should I wait a bit for more comments or straightforwardly submit
this as a patch rather than RFC ?

Mathieu



-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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