linux-kernel - Re: [PATCH 01/10] cpu: Introduce clear_tasks_mm

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Date:	Sat, 24 Mar 2012 13:43:41 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Anton Vorontsov <anton.vorontsov@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	Mike Frysinger <vapier@...too.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Richard Weinberger <richard@....at>,
	Paul Mundt <lethal@...ux-sh.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	John Stultz <john.stultz@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net, linux-mm@...ck.org
Subject: Re: [PATCH 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

On Sat, 2012-03-24 at 14:27 +0400, Anton Vorontsov wrote:
> +void clear_tasks_mm_cpumask(int cpu)
> +{
> +       struct task_struct *p;
> +
> +       read_lock(&tasklist_lock);
> +       for_each_process(p) {
> +               struct task_struct *t;
> +
> +               t = find_lock_task_mm(p);
> +               if (!t)
> +                       continue;
> +               cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
> +               task_unlock(t);
> +       }
> +       read_unlock(&tasklist_lock);
> +} 

Why bother with the tasklist_lock at all anymore, afaict you could use
rcu_read_lock() here. This all is called after the cpu is taken down and
marked offline, so its not like new tasks will ever get this cpu set in
their mm mask.


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