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Date: Wed, 12 Dec 2012 19:23:08 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc: tglx@...utronix.de, peterz@...radead.org,
paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU
offline from atomic context
On 12/12, Srivatsa S. Bhat wrote:
>
> On 12/12/2012 10:54 PM, Oleg Nesterov wrote:
>
> > And when I look at get_online_cpus_atomic() again it uses rmb(). This
> > doesn't look correct, we need the full barrier between this_cpu_inc()
> > and writer_active().
>
> Hmm..
>
> > At the same time reader_nested_percpu() can be checked before mb().
>
> I thought that since the increment and the check (reader_nested_percpu)
> act on the same memory location, they will naturally be run in the given
> order, without any need for barriers. Am I wrong?
And this is what I meant, you do not need a barrier before
reader_nested_percpu().
But you need to ensure that WRITE(reader_percpu_refcnt) and READ(writer_signal)
can't be reordered, so you need mb() in between. rmb() can serialize LOADs and
STOREs.
Or I misunderstood?
Oleg.
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