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Message-ID: <20090730175108.GC3617@redhat.com>
Date: Thu, 30 Jul 2009 19:51:08 +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] cpusets: fix deadlock with cpu_down()->cpuset_lock()
On 07/30, Lai Jiangshan wrote:
>
> Oleg Nesterov wrote:
> > On 07/29, Oleg Nesterov wrote:
> >> I strongly believe the bug does exist, but this patch needs the review
> >> from maintainers.
> >
> > Yes...
> >
> >> IOW, with this patch migration_call(CPU_DEAD) runs without callback_mutex,
> >> but kernel/cpuset.c always takes get_online_cpus() before callback_mutex.
> >
> > Oh. I'm afraid this is not an option.
> >
> > callback_mutex should nest under cgroup_mutex, but cpu hotplu pathes
> > take cgroup_mutex under cpu_hotplug->lock. Lockdep won't be happy.
> >
> > Oleg.
> >
>
> We have made great effort to remove get_online_cpus() from cgroup_mutex
> critical region.
Agreed.
> We can migrate the owner of callback_mutex in migration_call(CPU_DEAD)
> at first(and then take callback_mutex and migrate others).
Not sure I understand how can we do this. Even if we know the owner
of callback_mutex, if we can migrate it safely without callback_mutex
why we can't migrate other tasks without this lock?
In any case this doesn't look like a clean solution, imho. But I hardly
understand what cpuset is, can't suggest something clever.
I don't really understand why guarantee_online_cpus() needs this mutex,
and I don' understand why it have to check cs->parent.
update_cpumask() doesn't allow to set ->cpus_allowed which does not
intersect with cpu_online_mask (unless cs is empty). This means that
guarantee_online_cpus()->cpumask_intersects() == T is only possible
when we are called from cpu_down() path, right? But can't we just
return cpu_online_mask in this case? I mean,
static void guarantee_online_cpus(const struct cpuset *cs,
struct cpumask *pmask)
{
if (cpumask_intersects(cs->cpus_allowed, cpu_online_mask))
cpumask_and(pmask, cs->cpus_allowed, cpu_online_mask);
else
/* !!!!!!!!!!!!!!!!!!!!
* cpuset_track_online_cpus(CPU_DEAD)->scan_for_empty_cpusets()
* will fix this.
*/
cpumask_copy(pmask, cpu_online_mask);
}
Most probably I missed something, never looked in cpuset.c before.
Oleg.
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