linux-kernel - Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

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Message-ID: <1302656736.3981.138.camel@sli10-conroe>
Date:	Wed, 13 Apr 2011 09:05:36 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>
Subject: Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

On Wed, 2011-04-13 at 03:02 +0800, Tejun Heo wrote:
> On Tue, Apr 12, 2011 at 04:04:04PM +0800, Shaohua Li wrote:
> >  static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> >  {
> > +#if BITS_PER_LONG == 32
> > +	s64 count;
> > +	spin_lock(&fbc->lock);
> > +	count = fbc->count;
> > +	spin_unlock(&fbc->lock);
> > +	return count;
> > +#else
> >  	return fbc->count;
> > +#endif
> 
> I don't know.  Is there any problem caused by this?  The interface is
> known to be unreliable and already being used in speculative manner.
> I think it's more beneficial to avoid using locks on fast read path.
yes, it is unreliable, but only in an extent of batch*nr_cpus. accessing
64bits in 32bit machine can give us a _very_ big inaccuracy, which is
unacceptable to me.

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