linux-kernel - Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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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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