linux-kernel - Re: [PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm

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Date:	Sun, 25 Mar 2012 19:42:10 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Anton Vorontsov <anton.vorontsov@...aro.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask()
	helper

On 03/24, Anton Vorontsov wrote:
>
> Many architctures clear tasks' mm_cpumask like this:
>
> 	read_lock(&tasklist_lock);
> 	for_each_process(p) {
> 		if (p->mm)
> 			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
> 	}
> 	read_unlock(&tasklist_lock);

Namely arm, powerpc, and sh.

> The code above has several problems, such as:
>
> 1. Working with task->mm w/o getting mm or grabing the task lock is
>    dangerous as ->mm might disappear (exit_mm() assigns NULL under
>    task_lock(), so tasklist lock is not enough).

This is not actually true for arm and sh, afaics. They do not even
need tasklist or rcu lock for for_each_process().

__cpu_disable() is called by __stop_machine(), we know that nobody
can preempt us and other CPUs can do nothing.

> 2. Checking for process->mm is not enough because process' main
>    thread may exit or detach its mm via use_mm(), but other threads
>    may still have a valid mm.

Yes,

> Also, Per Peter Zijlstra's idea, now we don't grab tasklist_lock in
> the new helper, instead we take the rcu read lock. We can do this
> because the function is called after the cpu is taken down and marked
> offline, so no new tasks will get this cpu set in their mm mask.

And only powerpc needs rcu_read_lock() and task_lock().

OTOH, I do not understand why powepc does this on CPU_DEAD...
And probably CPU_UP_CANCELED doesn't need to clear mm_cpumask().

That said, personally I think these patches are fine, the common
helper makes sense.

Oleg.

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