linux-kernel - Re: [PATCH cgroup/for-3.14-fixes] cgroup: protect modifications to cgroup_idr with cgroup

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Date:	Wed, 12 Feb 2014 10:15:52 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH cgroup/for-3.14-fixes] cgroup: protect modifications to
 cgroup_idr with cgroup_mutex

On 2014/2/12 0:26, Michal Hocko wrote:
> On Tue 11-02-14 10:41:05, Tejun Heo wrote:
> [...]
>> @@ -4254,12 +4256,12 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
>>  
>>  	return 0;
>>  
>> -err_unlock:
>> -	mutex_unlock(&cgroup_mutex);
>> -	/* Release the reference count that we took on the superblock */
>> -	deactivate_super(sb);
>>  err_free_id:
>>  	idr_remove(&root->cgroup_idr, cgrp->id);
>> +	/* Release the reference count that we took on the superblock */
>> +	deactivate_super(sb);
>> +err_unlock:
>> +	mutex_unlock(&cgroup_mutex);
>>  err_free_name:
>>  	kfree(rcu_dereference_raw(cgrp->name));
>>  err_free_cgrp:
> 
> Do I have to change deactivate_super vs. mutex_unlock ordering in my
> backport for 3.12 as well?
> 

Your change is wrong that you shouldn't drop sb refcnt in err_unlock path.

But you made me think if it's OK to hold cgroup_mutex while calling deactivate_super(),
and the answer is NO! deactive_super() may call cgroup_kill_sb() which will
acquire cgroup_mutex.

I'll update the patch.

Thank Tejun we won't be entangled with vfs internal anymore after coverting
to kernfs.

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