linux-kernel - Re: [RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline

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Date:	Thu, 06 Dec 2012 00:31:14 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.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, oleg@...hat.com,
	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 v2 02/10] CPU hotplug: Provide APIs for "full" atomic
 readers to prevent CPU offline

Replaying what Tejun wrote:

On 12/06/2012 12:13 AM, Srivatsa S. Bhat wrote:
> Some of the atomic hotplug readers cannot tolerate CPUs going offline while
> they are in their critical section. That is, they can't get away with just
> synchronizing with the updates to the cpu_online_mask; they really need to
> synchronize with the entire CPU tear-down sequence, because they are very
> much involved in the hotplug related code paths.
> 
> Such "full" atomic hotplug readers need a way to *actually* and *truly*
> prevent CPUs from going offline while they are active.
> 

I don't think this is a good idea.  You really should just need
get/put_online_cpus() and get/put_online_cpus_atomic().  The former
the same as they are.  The latter replacing what
preempt_disable/enable() was protecting.  Let's please not go
overboard unless we know they're necessary.  I strongly suspect that
breaking up reader side from preempt_disable and making writer side a
bit lighter should be enough.  Conceptually, it really should be a
simple conversion - convert preempt_disable/enable() pairs protecting
CPU on/offlining w/ get/put_cpu_online_atomic() and wrap the
stop_machine() section with the matching write lock.

Thanks.

-- tejun 

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