linux-kernel - Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus

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Date:	Wed, 26 Jun 2013 07:34:24 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	peterz@...radead.org, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, walken@...gle.com, mingo@...nel.org,
	linux-arch@...r.kernel.org, vincent.guittot@...aro.org,
	xiaoguangrong@...ux.vnet.ibm.com, wangyun@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	rusty@...tcorp.com.au, rostedt@...dmis.org, namhyung@...nel.org,
	tglx@...utronix.de, laijs@...fujitsu.com, zhong@...ux.vnet.ibm.com,
	netdev@...r.kernel.org, oleg@...hat.com, sbw@....edu,
	tj@...nel.org, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to
 prevent CPU offline

On Wed, Jun 26, 2013 at 03:29:40PM +0100, David Laight wrote:
> > Once stop_machine() is gone from the CPU offline path, we won't be able
> > to depend on disabling preemption to prevent CPUs from going offline
> > from under us.
> 
> Could you use an rcu-like sequence so that disabling pre-emption
> would be enough?
> 
> Something like rebuilding the cpu list, then forcing yourself
> to run on all the cpu.
> 
> That would be far less intrusive.

It would also increase the latency of CPU-hotunplug operations.

							Thanx, Paul

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