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

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Message-ID: <20100301165038.GA14558@redhat.com>
Date:	Mon, 1 Mar 2010 17:50:38 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
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

On 03/02, Tejun Heo wrote:
>
> Hello,
>
> On 03/02/2010 12:37 AM, Oleg Nesterov wrote:
> >
> > 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().
>
> 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.

Agreed, a little comment can help.

But, just in case, I forgot to repeat this case is not possible anyway.
_cpu_down() ensures idle_cpu(cpu) == T after __stop_machine(), this means
that this SCHED_FIFO thread we are going to kthread_stop() later can't
be active.

Oleg.

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