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

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Message-ID: <20100301154144.GB11090@redhat.com>
Date:	Mon, 1 Mar 2010 16:41:44 +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, again.
>
> On 02/28/2010 11:34 PM, Oleg Nesterov wrote:
> > On 02/26, Tejun Heo wrote:
> >>
> >> @@ -164,19 +259,18 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
> >>  	idle.fn = chill;
> >>  	idle.data = NULL;
> >>
> >> +	smp_wmb();			/* -> stop_cpu()::set_current_state() */
> >> ...
> >> +	for_each_online_cpu(i)
> >> +		wake_up_process(*per_cpu_ptr(stop_machine_threads, i));
> >
> > Afaics, this smp_wmb() is not needed, wake_up_process() (try_to_wake_up)
> > should ensure we can't race with set_current_state() + check_condition.
> > It does, note the wmb() in try_to_wake_up().
>
> Yeap, the initial version was like that and it was awkward to explain
> in the comment in stop_cpu() so I basically put it there as a
> documentation anchor.

OK,

> Do you think removing it would be better?

No, I just wanted to understand what I have missed. This applies to all
my questions in this thread ;)

Thanks,

Oleg.

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