linux-kernel - Re: [PATCH 2.6.32-rc6] workqueue: fix race condition in schedule_on_each

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Date:	Fri, 13 Nov 2009 16:38:27 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2.6.32-rc6] workqueue: fix race condition in
	schedule_on_each_cpu()

On 11/13, Tejun Heo wrote:
>
> Commit 65a64464349883891e21e74af16c05d6e1eeb4e9 which allows
> schedule_on_each_cpu() to be called from keventd added a race
> condition.  schedule_on_each_cpu() may race with cpu hotplug and end
> up executing the function twice on a cpu.
>
> Fix it by moving direct execution into the section protected with
> get/put_online_cpus().  While at it, update code such that direct
> execution is done after works have been scheduled for all other cpus
> and drop unnecessary cpu != orig test from flush loop.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> ---
> Andi, Oleg, this patch tested fine on my machine but it would be great
> if you guys can ack it.  Ingo, upon ack, can you please route this
> patch?

Thanks, I think this is correct.

A very minor nit, schedule_on_each_cpu() still checks "orig" twice,
perhaps it makes sense to do

	for_each_online_cpu(cpu) {
		struct work_struct *work = per_cpu_ptr(works, cpu);

		INIT_WORK(work, func);

		if (likely(cpu != orig))
			schedule_work_on(cpu, work);
		else
			func(work);
	}

instead and simplify the code a little bit.

But this is minor and up to you.

Acked-by: Oleg Nesterov <oleg@...hat.com>

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