linux-kernel - Re: [thisops uV3 08/18] Taskstats: Use this_cpu

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Date:	Wed, 1 Dec 2010 12:13:44 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [thisops uV3 08/18] Taskstats: Use this_cpu_ops

On Wed, 1 Dec 2010, Michael Holzheu wrote:

> >  		return -ENOMEM;
> >
> >  	if (!info) {
> > -		int seq = get_cpu_var(taskstats_seqnum)++;
> > -		put_cpu_var(taskstats_seqnum);
> > +		int seq = this_cpu_inc_return(taskstats_seqnum);
>
> Hmmm, wouldn't seq now always be one more than before?
>
> I think that "seq = get_cpu_var(taskstats_seqnum)++" first assigns
> taskstats_seqnum to seq and then increases the value in contrast to
> this_cpu_inc_return() that returns the already increased value, correct?

Correct. We need to subtract one from that (which will eliminate the minus
-1 that the inline this_cpu_inc_return creates).

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