linux-kernel - Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

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Date:	Thu, 06 Dec 2012 00:21:06 +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 01/10] CPU hotplug: Provide APIs for "light" atomic
 readers to prevent CPU offline

Replaying what Oleg wrote:

Hi,

Sorry I don't understand the context and I can't find this thread
anywhere, so I am not sure I understand...

> Replaying what Tejun wrote:
> So, we basically need percpu_rwlock.  We already have percpu_rwsem.

Yes, and with -mm patches it becomes reader-friendly. In particular
see http://marc.info/?l=linux-mm-commits&m=135240650828875

> Oleg, it seems
> CPU hotplug needs big-reader rwlock, ideas on how to proceed?
> 

I am going to convert get_online_cpus() to use percpu_down_read(),
this looks simple.

We already discussed this with Paul, see

	http://marc.info/?l=linux-kernel&m=135248463226031

and the whole thread.

In short, all we need is percpu_down_write_recursive_readers() and
afaics the only complication is lockdep, we need down_read_no_lockdep()
which (like __up_read) doesn't do rwsem_acquire_read().

Oleg.

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