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Date: Tue, 4 Aug 2009 18:36:22 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
       Rusty Russell <rusty@rustcorp.com.au>, linux-kernel@vger.kernel.org,
       Li Zefan <lizf@cn.fujitsu.com>, Miao Xie <miaox@cn.fujitsu.com>,
       Paul Menage <menage@google.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Gautham R Shenoy <ego@in.ibm.com>
Subject: Re: [PATCH] cpusets: rework guarantee_online_cpus() to fix deadlock
	with cpu_down()
Message-ID: <20090804163622.GA6999@redhat.com>
References: <20090729212125.GA16970@redhat.com> <20090729212216.GB16970@redhat.com> <20090729230043.GA28175@redhat.com> <4A70FD26.1010800@cn.fujitsu.com> <20090730175108.GC3617@redhat.com> <4A725594.8020205@cn.fujitsu.com> <20090801044236.GA23975@redhat.com> <4A74F767.7060401@cn.fujitsu.com> <20090802065504.GA27164@redhat.com> <4A77E4D0.20900@cn.fujitsu.com>
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On 08/04, Lai Jiangshan wrote:
>
> Oleg Nesterov wrote:
> > On 08/02, Lai Jiangshan wrote:
> >> Oleg Nesterov wrote:
> >>
> >>> 	- do NOT scan cs->parent cpusets. If there are no online CPUs in
> >>> 	  cs->cpus_allowed, we use cpu_online_mask. This is only possible
> >>> 	  when we are called by cpu_down() hooks, in that case
> >>> 	  cpuset_track_online_cpus(CPU_DEAD) will fix things later.
> >>>
> >>
> >> We must scan cs->parent cpusets.
> >> A task is constrained by a cpuset,
> >
> > Yes, the task esacpes its cpuset. With or without this patch.
> > Because cs->cpus_allowed has no online CPUs.
> >
> >> it must be constrained
> >> this cpuset's parent too.
> >
> > It will be constained again, after scan_for_empty_cpusets(), no?
>
> cpuset_cpus_allowed() is not only  used for CPU offline.
>
> sched_setaffinity() also uses it.

Sure. And it must take get_online_cpus() to avoid the races with hotplug.

> The task will not be constained again.

It will be constrained, and we don't need to scan cs->parent.

Because, since we are protected from cpu_down(), cs->cpus_allowed must
have online CPUs. update_cpumask() doesn't allow the empty cpumasks.

> Or I missed something.

Or me ;)

Oleg.