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 18:24:31 +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, Oleg Nesterov wrote:
>
> On 12/11, Srivatsa S. Bhat wrote:
> >
> > IOW, the hotplug readers just increment/decrement their per-cpu refcounts
> > when no writer is active.
>
> plus cli/sti ;) and increment/decrement are atomic.
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

OOPS, sorry I was going to say "adds mb()".

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().

At the same time reader_nested_percpu() can be checked before mb().

Oleg.

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