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Date: Sat, 6 Oct 2007 10:54:35 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
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To: Paul Jackson <pj@sgi.com>
cc: Paul Menage <menage@google.com>, Andrew Morton <akpm@linux-foundation.org>,
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       svaidy@linux.vnet.ibm.com, xemul@openvz.org, containers@lists.osdl.org,
       balbir@linux.vnet.ibm.com
Subject: Re: [PATCH] task containersv11 add tasks file interface fix for
 cpusets
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On Sat, 6 Oct 2007, Paul Jackson wrote:

> This isn't working for me.
> 
> The key kernel routine for updating a tasks cpus_allowed
> cannot be called while holding a spinlock.
> 
> But the above loop holds a spinlock, css_set_lock, between
> the cgroup_iter_start and the cgroup_iter_end.
> 
> I end up generating complaints of:
> 	BUG: scheduling while atomic
> when I invoke the set_cpus_allowed() above.
> 
> Should css_set_lock be a mutex?  Locking changes like that
> can be risky.
> 

It would probably be better to just save references to the tasks.

	struct cgroup_iter it;
	struct task_struct *p, **tasks;
	int i = 0;

	cgroup_iter_start(cs->css.cgroup, &it);
	while ((p = cgroup_iter_next(cs->css.cgroup, &it))) {
		get_task_struct(p);
		tasks[i++] = p;
	}
	cgroup_iter_end(cs->css.cgroup, &it);

	while (--i >= 0) {
		set_cpus_allowed(tasks[i], cs->cpus_allowed);
		put_task_struct(tasks[i]);
	}

The getting and putting of the tasks will prevent them from exiting or 
being deallocated prematurely.  But this is also a critical section that 
will need to be protected by some mutex so it doesn't race with other 
set_cpus_allowed().

		David