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

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Message-ID: <CAOS58YOQNs9RcNAXHKfkwaxMeDeSzOczNC=-WS5uvsas168T+A@mail.gmail.com>
Date:	Mon, 28 Jan 2013 12:27:45 -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

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()?

> /* 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)?

Thanks.

-- 
tejun
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