linux-kernel - Re: get/put_online_cpus documentation wrong?

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Message-ID: <4F9FEC10.1080102@linux.vnet.ibm.com>
Date:	Tue, 01 May 2012 19:28:40 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Paul Mackerras <paulus@...ba.org>
CC:	Gilad Ben-Yossef <gilad@...yossef.com>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: get/put_online_cpus documentation wrong?

On 05/01/2012 04:56 PM, Gilad Ben-Yossef wrote:

> On Tue, May 1, 2012 at 3:12 AM, Paul Mackerras <paulus@...ba.org> wrote:
>>
>> In Documentation/cpu-hotplug.txt it says:
>>
>> "       #include <linux/cpu.h>
>>        get_online_cpus() and put_online_cpus():
>>
>> The above calls are used to inhibit cpu hotplug operations. While the
>> cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
>> If you merely need to avoid cpus going away, you could also use
>> preempt_disable() and preempt_enable() for those sections.
>> Just remember the critical section cannot call any
>> function that can sleep or schedule this process away."
> 
> 
> I *think* the critical section remark here is referring to using
> preempt_disable/enable(),
> rather  then the use of get/put_online_cpus().
>

Yes, what Gilad said is right.

Using preempt_disable()/preempt_enable() around a piece of code is a
light-weight way of preventing CPUs from going away (CPUs going offline)
while executing that code. (However, they don't prevent new CPUs from
coming online). So, if you are using preempt_disable|enable() to
prevent some CPU from going offline, then the usual rules for using
preempt_disable|enable() apply - that code should not sleep.

But if you want to avoid CPU hotplug entirely (both CPU offlining and
onlining), then you should use get/put_online_cpus(). You can use this
around any piece of code, including those which can sleep.

Regards,
Srivatsa S. Bhat
IBM Linux Technology Center

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