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

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Message-ID: <20080814014944.GA31883@Krystal>
Date:	Wed, 13 Aug 2008 21:49:44 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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 (jeremy@...p.org) wrote:
> H. Peter Anvin wrote:
>> I believe this should be okay.  In 32-bit mode some of the security and 
>> hypervisor frameworks want to set segment limits, but I don't believe they 
>> ever would set DS and SS inconsistently, or that we'd handle a #GP versus 
>> an #SS differently (segment violations on the stack segment are #SS, not 
>> #GP.)  To be 100% sure we'd have to pick apart the modr/m byte to figure 
>> out what the base register is but I think that's total overkill.
>
> The kernel sets ds and ss to the same selector, so they're always going to 
> have the same underlying descriptor.
>
> My only concern is whether there are any locked instructions which are 
> explicitly using a cs: override for those odd corners of the kernel.  I 
> don't think so.
>
> That said, I wonder how useful it is to do the SMP->UP code transition.  
> How often does a kernel go from being SMP to UP in a situation where we 
> really care about the performance?  And that won't be shortly be becoming 
> SMP again anyway?
>

A virtualized guest kernel could use that to limit its use of the
overall machine CPU resources in different time periods. Policies can
determine how many physical CPU a virtual machine can be tied to, and
that may change depending on e.g. the workload or time of day. Having
the ability to efficiently switch to UP for a long period of time seems
important in this use-case.

Mathieu

>    J

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