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

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Date:	Sat, 6 Oct 2007 13:53:53 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Paul Jackson" <pj@....com>
Cc:	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,
	rientjes@...gle.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 10/6/07, Paul Jackson <pj@....com> 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.

css_set_lock is an rwlock currently; I'd rather not turn it into an rw
mutex since there are places that it gets taken where we can't afford
to sleep.

>
> Or perhaps there should be another callback, called only by
> attach() requests back to the same group.  Likely cpusets would
> be the only subsystem interested in plugging that callback.
>
> That, or my original patch, which calls the attach routine
> even if re-attaching to the current cgroup ...

I'd prefer David's solution of grabbing references to tasks during the
iteration and then doing set_cpus_allowed outside the tasklist_lock.

Paul
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