Re: [PATCH] generic dynamic per cpu refcounting

[Tejun Heo]

Hello, Kent.

On Mon, Jan 28, 2013 at 12:55:40PM -0800, Kent Overstreet wrote:
> > I don't understand why we need two stages. What prevents the killing
> > thread from fetching percpu counters after dying passes one
> > synchronize_sched()?
> 
> It does. The second synchronize_sched() is needed after we set state :=
> dead, and before we drop the initial ref. Otherwise the ref could hit 0
> before percpu_ref_put knows to check for it.

Still a bit confused.  Why do we need to make the two steps separate?
What prevents us from doing the following?

	set dying;
	synchronize_sched();
	collect percpu refs into global atomic_t;
	put the base ref;

Thanks.

-- 
tejun
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