Message-ID: <48A47B83.3090408@zytor.com>
Date: Thu, 14 Aug 2008 11:37:55 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Andi Kleen <andi@firstfloor.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
       Steven Rostedt <srostedt@redhat.com>,
       LKML <linux-kernel@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       David Miller <davem@davemloft.net>, Roland McGrath <roland@redhat.com>,
       Ulrich Drepper <drepper@redhat.com>,
       Rusty Russell <rusty@rustcorp.com.au>,
       Gregory Haskins <ghaskins@novell.com>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       "Luis Claudio R. Goncalves" <lclaudio@uudg.org>,
       Clark Williams <williams@redhat.com>,
       Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with	preemptible
 kernel and CPU hotplug
References: <20080813200119.GA18966@Krystal> <20080813234156.GA25775@Krystal> <48A375E3.9090609@zytor.com> <48A388CE.2020404@goop.org> <20080814014944.GA31883@Krystal> <48A3A806.8060509@goop.org> <20080814151805.GA29507@Krystal> <48A459B1.2070601@zytor.com> <20080814165802.GC517@Krystal> <48A465F2.8000701@goop.org> <20080814173021.GA4697@Krystal> <48A46EC2.1010301@goop.org>
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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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