linux-kernel - Re: [PATCH 04/21] Generic percpu refcounting

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Date:	Tue, 14 May 2013 17:28:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Kent Overstreet <koverstreet@...gle.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-aio@...ck.org, akpm@...ux-foundation.org,
	Zach Brown <zab@...hat.com>, Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>,
	Asai Thambi S P <asamymuthupa@...ron.com>,
	Selvan Mani <smani@...ron.com>,
	Sam Bradshaw <sbradshaw@...ron.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 04/21] Generic percpu refcounting

On 05/14, Tejun Heo wrote:
>
> > +int percpu_ref_tryget(struct percpu_ref *ref)
> > +{
> > +	int ret = 1;
> > +
> > +	preempt_disable();
> > +
> > +	if (!percpu_ref_dead(ref))
> > +		percpu_ref_get(ref);
> > +	else
> > +		ret = 0;
> > +
> > +	preempt_enable();
> > +
> > +	return ret;
> > +}
...
> BTW, why is this
> function necessary?  What's the use case?

Yes, I was wondering too.

And please note that this code _looks_ wrong, percpu_ref_get() still
can increment ref->count.

Hmm. Just noticed this comment above percpu_ref_kill()

	* The caller must issue a synchronize_rcu()/call_rcu() before calling
	* percpu_ref_put() to drop the initial ref.

Really?

Oleg.

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