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

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Message-ID: <20130515090053.GC16164@moria.home.lan>
Date:	Wed, 15 May 2013 02:00:53 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, 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 Tue, May 14, 2013 at 05:28:36PM +0200, Oleg Nesterov wrote:
> 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.

Yeah I see what you mean, I changed how ret is set.

But also splitting tryget() and count() out into another patch to go
with the module conversion.

> 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?

That's also left over from the dynamic version, whoops.
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