linux-kernel - Re: [PATCH 1/1] cpu_hotplug: don't play with current->cpus

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Date:	Thu, 30 Jul 2009 18:32:58 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [PATCH 1/1] cpu_hotplug: don't play with current->cpus_allowed

On 07/30, Lai Jiangshan wrote:
>
> Oleg Nesterov wrote:
> > _cpu_down() changes the current task's affinity and then recovers it at
> > the end. The problems are well known: we can't restore old_allowed if it
> > was bound to the now-dead-cpu, and we can race with the userspace which
> > can change cpu-affinity during unplug.
> >
> > _cpu_down() should not play with current->cpus_allowed at all. Instead,
> > take_cpu_down() can migrate the caller of _cpu_down() after __cpu_disable()
> > removes the dying cpu from cpu_online_mask.
> >
> >  static int __ref take_cpu_down(void *_param)
> >  {
> >  	struct take_cpu_down_param *param = _param;
> > +	unsigned int cpu = (unsigned long)param->hcpu;
> >  	int err;
> >
> >  	/* Ensure this CPU doesn't handle any more interrupts. */
> > @@ -181,6 +183,8 @@ static int __ref take_cpu_down(void *_pa
> >  	raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
> >  				param->hcpu);
> >
> > +	if (task_cpu(param->caller) == cpu)
> > +		move_task_off_dead_cpu(cpu, param->caller);
>
> move_task_off_dead_cpu() calls cpuset_cpus_allowed_locked() which
> needs callback_mutex held. But actually we don't hold it, it'll
> will corrupt the work of other task which holds callback_mutex.
> Is it right?

Of course it is not. That is why I tried to kill cpuset_lock() first.

And I still think it must die. But I don't know how to remove it.

Oleg.

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