linux-kernel - Re: [PATCH] task containersv11 add tasks file interface fix for cpusets

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Date:	Sat, 6 Oct 2007 23:15:14 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Paul Menage <menage@...gle.com>
cc:	Paul Jackson <pj@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	nickpiggin@...oo.com.au, a.p.zijlstra@...llo.nl, serue@...ibm.com,
	clg@...ibm.com, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com, svaidy@...ux.vnet.ibm.com, xemul@...nvz.org,
	containers@...ts.osdl.org, balbir@...ux.vnet.ibm.com
Subject: Re: [PATCH] task containersv11 add tasks file interface fix for
 cpusets

On Sat, 6 Oct 2007, Paul Menage wrote:

> > 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().
> 
> Is that necessary? If some other process calls set_cpus_allowed()
> concurrently with a cpuset cpus update, it's not clear that there's
> any defined serialization semantics that have to be achieved, as long
> as the end result is that the task's cpus_allowed are within the
> cpuset's cpus_allowed.
> 

It can race with sched_setaffinity().  It has to give up tasklist_lock as 
well to call set_cpus_allowed() and can race

	cpus_allowed = cpuset_cpus_allowed(p);
	cpus_and(new_mask, new_mask, cpus_allowed);
	retval = set_cpus_allowed(p, new_mask);

and allow a task to have a cpu outside of the cpuset's new cpus_allowed if 
you've taken it away between cpuset_cpus_allowed() and set_cpus_allowed().

		David
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