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

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Date:	Mon, 28 Jan 2013 13:28:14 -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

On Mon, Jan 28, 2013 at 01:24:07PM -0800, Kent Overstreet wrote:
> > 	set dying;
> > 	synchronize_sched();
> > 	collect percpu refs into global atomic_t;
> > 	put the base ref;
> 
> After you set state := dying, percpu_ref_put() decrements the atomic_t,
> but it can't check if it's 0 yet because the thread that's collecting
> the percpu refs might not be done yet.
> 
> So percpu_ref_put can't check for ref == 0 until after state == dead.
> But the put in your example might have made ref 0. When did you set
> state to dead?

But at that point, the operation is already global, so there gotta be
a lighter way to synchronize stuff than going through full grace
period.  ie. You can add a bias value before marking dead so that the
counter never reaches zero before all percpu counters are collected
and then unbias it right before putting the base ref, that way the
only way you can hit zero ref is all refs are actually zero.

Thanks.

-- 
tejun
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