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

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Message-ID: <50C8C8C9.2070605@linux.vnet.ibm.com>
Date:	Wed, 12 Dec 2012 23:41:21 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.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 12/12/2012 10:54 PM, Oleg Nesterov wrote:
> 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()".
> 

Ok, got it :)

> 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?

(I referred Documentation/memory-barriers.txt again to verify this, and
the second point under the "Guarantees" section looked like it said the
same thing : "Overlapping loads and stores within a particular CPU will
appear to be ordered within that CPU").

Regards,
Srivatsa S. Bhat

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