linux-kernel - Re: [PATCH] Workqueue lockup: Circular dependency in threads

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Date:   Wed, 6 Sep 2017 16:47:25 +0530
From:   Prateek Sood <prsood@...eaurora.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     lizefan@...wei.com, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, sramana@...eaurora.org,
        mingo@...nel.org, longman@...hat.com, apkm@...ux-foundation.org
Subject: Re: [PATCH] Workqueue lockup: Circular dependency in threads

On 09/05/2017 06:52 PM, Tejun Heo wrote:
> Hello,
> 
> On Thu, Aug 31, 2017 at 06:43:56PM +0530, Prateek Sood wrote:
>>> 6) cpuset_mutex is acquired by task init:1 and is waiting for cpuhotplug lock.
> 
> Yeah, this is the problematic one.
> 
>>> We can reorder the sequence of locks as in the below diff to avoid this
>>> deadlock. But I am looking for inputs/better solution to fix this deadlock.
>>>
>>> ---
>>> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
>>>  /**
>>>   * update_tasks_cpumask - Update the cpumasks of tasks in the cpuset.
>>>   * @cs: the cpuset in which each task's cpus_allowed mask needs to be changed
>>> @@ -930,7 +946,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
>>>  	rcu_read_unlock();
>>>  
>>>  	if (need_rebuild_sched_domains)
>>> -		rebuild_sched_domains_locked();
>>> +		rebuild_sched_domains_unlocked()(without taking cpuhotplug.lock)
>>>  }
>>>  
>>>  /**
>>> @@ -1719,6 +1735,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>>> +	get_online_cpus();
>>>  	mutex_lock(&cpuset_mutex);
>>>  	if (!is_cpuset_online(cs))
>>>  		goto out_unlock;
>>> @@ -1744,6 +1761,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>>>  	mutex_unlock(&cpuset_mutex);
>>> +	put_online_cpus();
>>>  	kernfs_unbreak_active_protection(of->kn);
>>>  	css_put(&cs->css);
>>>  	flush_workqueue(cpuset_migrate_mm_wq);
>>>
> 
> And the patch looks good to me.  Can you please format the patch with
> proper description and sob?
> 
> Thanks.
> 

Thanks for review Tejun

I will send updated patch.

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