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Subject: Re: [PATCH] memcg: remove unneeded preempt_disable
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Lameter <cl@linux.com>
Cc: James Bottomley <James.bottomley@HansenPartnership.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Balbir Singh <bsingharora@gmail.com>,
        Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
        linux-arch@vger.kernel.org
Date: Fri, 26 Aug 2011 00:56:27 +0200
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On Thu, 2011-08-25 at 14:19 -0500, Christoph Lameter wrote:
> On Thu, 25 Aug 2011, Peter Zijlstra wrote:
> 
> > Also, I thought this_cpu thing's were at best locally atomic. If you
> > make them full blown atomic ops then even __this_cpu ops will have to be
> > full atomic ops, otherwise:
> >
> >
> > CPU0			CPU(1)
> >
> > this_cpu_inc(&foo);	preempt_disable();
> > 			__this_cpu_inc(&foo);
> > 			preempt_enable();
> >
> > might step on each other's toes.
> 
> They would both have their own instance of "foo". per cpu atomicity is
> only one requirement of this_cpu_ops. The other is the ability to relocate
> accesses relative to the current per cpu area.

Ah, but not if the this_cpu_inc() thing ends up being more than a single
instruction, then you have preemption/migration windows. Only when LL/SC
can deal with SC having a different EA from the LL and supports a big
enough offset could this possibly work.