linux-kernel - Re: [PATCH] cgroup/cpuset: remove circular dependency deadlock

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Date:   Thu, 7 Sep 2017 14:37:15 +0530
From:   Prateek Sood <prsood@...eaurora.org>
To:     Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     tj@...nel.org, lizefan@...wei.com, cgroups@...r.kernel.org,
        mingo@...nel.org, longman@...hat.com, linux-kernel@...r.kernel.org,
        sramana@...eaurora.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] cgroup/cpuset: remove circular dependency deadlock

On 09/07/2017 02:26 PM, Boqun Feng wrote:
> On Thu, Sep 07, 2017 at 09:28:48AM +0200, Peter Zijlstra wrote:
>> On Thu, Sep 07, 2017 at 11:34:12AM +0530, Prateek Sood wrote:
>>> Remove circular dependency deadlock in a scenario where hotplug of CPU is
>>> being done while there is updation in cgroup and cpuset triggered from
>>> userspace.
>>>
>>> Example scenario:
>>> kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
>>>
>>> kworker/0:0 - percpu_down_write(&cpu_hotplug_lock)  [held]
>>>               flush(work)   [no high prio workqueue available on CPU]
>>>               wait_for_completion()
> 
> Hi Prateek,
> 
> so this is:
> 
> 	_cpu_down():
> 	  cpus_write_lock(); // percpu_down_write(&cpu_hotlug_lock)
> 	  cpuhp_invoke_callbacks():
> 	    workqueue_offine_cpu():
> 	      wq_update_unbound_numa():
> 	        alloc_unbound_pool():
> 	          get_unbound_pool():
> 		    create_worker():
> 		      kthread_create_on_node():
> 		        wake_up_process(kthreadd_task);
> 		        wait_for_completion(); // create->done
> 
> , right?
> 
> Wonder running in a kworker is necessary to trigger this, I mean running
> a cpu_down() in a normal process context could also trigger this, no?
> Just ask out of curiosity.
> 
> Regards,
> Boqun

Hi Boqun,

cpu_down() in normal process can also trigger this.



Regards
Prateek
> 
>>>
>>> kthreadd    - percpu_down_read(cgroup_threadgroup_rwsem)  [waiting]
>>>
>>> init:729    - percpu_down_write(cgroup_threadgroup_rwsem)   [held]
>>>               lock(cpuset_mutex)   [waiting]
>>>
>>> init:1      - lock(cpuset_mutex)   [held]
>>>               percpu_down_read(&cpu_hotplug_lock)   [waiting]
>>
>> That's both unreadable and useless :/ You want to tell what code paths
>> that were, not which random tasks happened to run them.
>>
>>
> [...]
> 


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