linux-kernel - Re: [PATCH 22/45] percpu_counter: Use get/put_online_cpus

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Date:	Mon, 24 Jun 2013 23:39:58 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	tglx@...utronix.de, peterz@...radead.org, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	walken@...gle.com, vincent.guittot@...aro.org,
	laijs@...fujitsu.com, rostedt@...dmis.org,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	sbw@....edu, fweisbec@...il.com, zhong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 22/45] percpu_counter: Use get/put_online_cpus_atomic()
 to prevent CPU offline

On 06/24/2013 11:36 PM, Tejun Heo wrote:
> On Mon, Jun 24, 2013 at 10:55:35AM -0700, Tejun Heo wrote:
>>> @@ -105,6 +106,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)
>>>  		ret += *pcount;
>>>  	}
>>>  	raw_spin_unlock(&fbc->lock);
>>> +	put_online_cpus_atomic();
>>
>> I don't think this is necessary.  CPU on/offlining is explicitly
>> handled via the hotplug callback which synchronizes through fbc->lock.
>> __percpu_counter_sum() racing with actual on/offlining doesn't affect
>> correctness and adding superflous get_online_cpus_atomic() around it
>> can be misleading.
> 
> Ah, okay, so you added a debug feature which triggers warning if
> online mask is accessed without synchronization.

Exactly!

>  Yeah, that makes
> sense and while the above is not strictly necessary, it probably is
> better to just add it rather than suppressing the warning in a
> different way.

Yeah, I was beginning to scratch my head as to how to suppress the
warning after I read your explanation as to why the calls to
get/put_online_cpus_atomic() would be superfluous in this case...

But as you said, simply invoking those functions is much simpler ;-)

>  Can you please at least add a comment explaining that?
> 

Sure, will do. Thanks a lot Tejun!
 
Regards,
Srivatsa S. Bhat

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