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

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Message-ID: <20130515082109.GA16164@moria.home.lan>
Date:	Wed, 15 May 2013 01:21:09 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	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>, Tejun Heo <tj@...nel.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 03:51:01PM +0200, Oleg Nesterov wrote:
> On 05/13, Kent Overstreet wrote:
> >
> > +int percpu_ref_kill(struct percpu_ref *ref)
> > +{
> > +	unsigned __percpu *pcpu_count;
> > +	unsigned __percpu *old;
> > +	unsigned count = 0;
> > +	int cpu;
> > +
> > +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> > +
> > +	do {
> > +		if (!pcpu_count)
> > +			return 0;
> > +
> > +		old = pcpu_count;
> > +		pcpu_count = cmpxchg(&ref->pcpu_count, old, NULL);
> > +	} while (pcpu_count != old);
> 
> This is purely cosmetic, feel free to ignore. But afaics all we
> need is
> 
> 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> 	if (!cmpxchg(&ref->pcpu_count, pcpu_count, NULL))
> 		return 0;

Whoops, yep. I was ripping out the dynamic stuff from my dynamic percpu
refcount code and missed that bit.
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