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

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Message-ID: <20130128211832.GK22465@mtj.dyndns.org>
Date:	Mon, 28 Jan 2013 13:18:32 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
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

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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