linux-kernel - Re: [PATCH] generic dynamic per cpu refcounting

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Date:	Mon, 28 Jan 2013 12:55:40 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, srivatsa.bhat@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] generic dynamic per cpu refcounting

On Mon, Jan 28, 2013 at 12:27:45PM -0800, Tejun Heo wrote:
> Hey, Kent.
> 
> On Mon, Jan 28, 2013 at 12:22 PM, Kent Overstreet
> <koverstreet@...gle.com> wrote:
> > Could do that too, but then teardown gets really messy for the user - we
> > need two synchronize_rcu()s:
> >
> > state := dying
> >
> > synchronize_rcu()
> >
> > /* Now nothing's changing the per cpu counters */
> >
> > Add per cpu counters to atomic counter counter
> >
> > /* Atomic counter is now consistent */
> >
> > state := dead
> >
> > synchronize_rcu()
> 
> 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.

> 
> > /* Now percpu_ref_put will check for ref == 0 */
> >
> > /* Drop initial ref */
> >
> > percpu_ref_put()
> >
> > And note that the first synchronize_rcu() is only needed when we had
> > allocated per cpu counters, my current code skips it otherwise.
> 
> And regardless, at the interface level, can't it just provide
> percpu_ref_put_base_ref(release_fn)?

Yeah, can definitely provide one that wraps it all together.
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