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Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency
To: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, avagin@gmail.com,
        mingo@kernel.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, sramana@codeaurora.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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From: Prateek Sood <prsood@codeaurora.org>
Message-ID: <3c9b2a2d-ede4-1242-418a-353ec9f78db3@codeaurora.org>
Date: Mon, 15 Jan 2018 17:32:18 +0530
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On 01/02/2018 09:46 PM, Tejun Heo wrote:
> Hello,
> 
> On Fri, Dec 29, 2017 at 02:07:16AM +0530, Prateek Sood wrote:
>> task T is waiting for cpuset_mutex acquired
>> by kworker/2:1
>>
>> sh ==> cpuhp/2 ==> kworker/2:1 ==> sh 
>>
>> kworker/2:3 ==> kthreadd ==> Task T ==> kworker/2:1
>>
>> It seems that my earlier patch set should fix this scenario:
>> 1) Inverting locking order of cpuset_mutex and cpu_hotplug_lock.
>> 2) Make cpuset hotplug work synchronous.
>>
>> Could you please share your feedback.
> 
> Hmm... this can also be resolved by adding WQ_MEM_RECLAIM to the
> synchronize rcu workqueue, right?  Given the wide-spread usages of
> synchronize_rcu and friends, maybe that's the right solution, or at
> least something we also need to do, for this particular deadlock?
> 
> Again, I don't have anything against making the domain rebuliding part
> of cpuset operations synchronous and these tricky deadlock scenarios
> do indicate that doing so would probably be beneficial.  That said,
> tho, these scenarios seem more of manifestations of other problems
> exposed through kthreadd dependency than anything else.
> 
> Thanks.
> 

Hi TJ,

Thanks for suggesting WQ_MEM_RECLAIM solution.

My understanding of WQ_MEM_RECLAIM was that it needs to be used for
cases where memory pressure could cause deadlocks.

In this case it does not seem to be a memory pressure issue.
Overloading WQ_MEM_RECLAIM usage for solution to another problem
is the correct approach?

This scenario can be resolved by using WQ_MEM_RECLAIM and a separate
workqueue for rcu. But there seems to be a possibility in future if
any cpu hotplug callbacks use other predefined workqueues which do not
have WQ_MEM_RECLAIM option.

Please let me know your feedback on this.


Thanks


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