Re: [PATCH] memcg: remove unneeded preempt_disable

Peter Zijlstra
2011-08-25T22:56:55Z

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.