linux-kernel - Re: [PATCH 1/4]percpu_counter: make API return consistent value

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Date:	Wed, 13 Apr 2011 12:08:31 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>
Subject: Re: [PATCH 1/4]percpu_counter: make API return consistent value

Hello,

On Wed, Apr 13, 2011 at 09:24:15AM +0800, Shaohua Li wrote:
> > Ummm, on UP, the counters cannot be positive.
> s/positive/negative?

Yeap.

> >   The _positive interface
> > is there to make it easier to cope with deviations introduced by
> > unsynchronized modifications by different CPUs.  On UP, such
> > deviations don't happen at all so _positive interface is the same as
> > the counterpart without the postfix.
> I'm confused. the counter could be negative, we have *_dec, *_sub.

Yes it can technically but I was referring to the intent of the API.
The whole percpu counter is supposed to track a positive number.  The
_positive interface is there just to cope with deviations caused by
distribution over multiple per-cpu counters, which can't happen on UP.
When the counter can actually be negative, the API doesn't make whole
lot of sense.  I agree it's poorly documented.

Thanks.

-- 
tejun
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