linux-kernel - Re: [PATCH 4/9] net: openvswitch: use this_cpu

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Date:	Fri, 2 Nov 2012 14:01:13 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Jesse Gross <jesse@...ira.com>
cc:	Shan Wei <shanwei88@...il.com>, dev@...nvswitch.org,
	NetDev <netdev@...r.kernel.org>,
	Kernel-Maillist <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 4/9] net: openvswitch: use this_cpu_ptr per-cpu helper

On Thu, 1 Nov 2012, Jesse Gross wrote:

> On Thu, Nov 1, 2012 at 7:33 AM, Christoph Lameter <cl@...ux.com> wrote:
> > On Thu, 1 Nov 2012, Shan Wei wrote:
> >
> >> But for different field in same per-cpu variable, how to guarantee n_missed
> >> and n_hit are from same cpu?
> >> this_cpu_read(dp->stats_percpu->n_missed);
> >> [processor changed]
> >> this_cpu_read(dp->stats_percpu->n_hit);
> >
> > What does current guarantee that? If it is guaranteed then you can use the
> > __this_cpu_xxx ops.
>
> Preemption is disabled in all of the places where writes are done and
> all of the reads are from foreign CPUs.

Since preemption is disabled no processor change can occur. So its safe to
use __this_cpu ops throughout and they will operate on the current per cpu
area.

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