Re: [PATCH] memcg: remove unneeded preempt_disable

Valdis.Kletnieks
2011-08-18T18:28:11Z

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On Thu, 18 Aug 2011 16:41:53 +0200, Johannes Weiner said:
> On Thu, Aug 18, 2011 at 10:26:58AM -0400, Valdis.Kletnieks@vt.edu wrote:
> > On Thu, 18 Aug 2011 11:38:00 +0200, Johannes Weiner said:
> >
> > > Note that on non-x86, these operations themselves actually disable and
> > > reenable preemption each time, so you trade a pair of add and sub on
> > > x86
> > >
> > > - preempt_disable()
> > > __this_cpu_xxx()
> > > __this_cpu_yyy()
> > > - preempt_enable()
> > >
> > > with
> > >
> > > preempt_disable()
> > > __this_cpu_xxx()
> > > + preempt_enable()
> > > + preempt_disable()
> > > __this_cpu_yyy()
> > > preempt_enable()
> > >
> > > everywhere else.
> >
> > That would be an unexpected race condition on non-x86, if you expected _xxx and
> > _yyy to be done together without a preempt between them. Would take mere
> > mortals forever to figure that one out. :)
>
> That should be fine, we don't require the two counters to be perfectly
> coherent with respect to each other, which is the justification for
> this optimization in the first place.

I meant the general case - when reviewing code, I wouldn't expect 2 lines of code
wrapped in preempt disable/enable to have a preempt window in the middle. ;)

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