linux-kernel - Re: [PATCH 10/43] stop_machine: reimplement without using workqueue

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Date:	Tue, 02 Mar 2010 01:36:25 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	torvalds@...ux-foundation.org, mingo@...e.hu, peterz@...radead.org,
	awalls@...ix.net, linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org
Subject: Re: [PATCH 10/43] stop_machine: reimplement without using workqueue

Hello,

On 03/02/2010 12:37 AM, Oleg Nesterov wrote:
>> Hmmm... I'm probably missing something but I don't see how
>> stop_machine_cpu_callback(CPU_POST_DEAD) depends on stop_cpu() thread
>> already parked in schedule().  Can you elaborate a bit?
> 
> Suppose that, when stop_machine_cpu_callback(CPU_POST_DEAD) is called,
> that stop_cpu() thread T is still running and it is going to check state
> before schedule().
> 
> CPU_POST_DEAD is called after cpu_hotplug_done(), another CPU can do
> stop_machine() and set STOPMACHINE_PREPARE.
> 
> If T sees state == STOPMACHINE_PREPARE it will join the game, but it
> wasn't counted in thread_ack counter, it is not cpu-bound, etc.

Oh, I see.  I was thinking get/put_online_cpus() block is exclusive
against cpu_maps_update_begin/done() instead of
cpu_hotplug_begin/done().  Will update and add comments.

Thanks.

-- 
tejun
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