Subject: Re: [PATCH v2 4/5] don't take cgroup_mutex in destroy()

From: Li Zefan <[email-masked]>
Date: Wed, 25 Apr 2012 16:01:03 +0800

Glauber Costa wrote:

> On 04/23/2012 11:31 PM, KAMEZAWA Hiroyuki wrote:
>> (2012/04/24 4:37), Glauber Costa wrote:
>>
>>> Most of the destroy functions are only doing very simple things
>>> like freeing memory.
>>>
>>> The ones who goes through lists and such, already use its own
>>> locking for those.
>>>
>>> * The cgroup itself won't go away until we free it, (after destroy)
>>> * The parent won't go away because we hold a reference count
>>> * There are no more tasks in the cgroup, and the cgroup is declared
>>>    dead (cgroup_is_removed() == true)
>>>
>>> [v2: don't cgroup_lock the freezer and blkcg ]
>>>
>>> Signed-off-by: Glauber Costa<[email-masked]>
>>> CC: Tejun Heo<[email-masked]>
>>> CC: Li Zefan<[email-masked]>
>>> CC: Kamezawa Hiroyuki<[email-masked]>
>>> CC: Vivek Goyal<[email-masked]>
>>> ---
>>>   kernel/cgroup.c |    9 ++++-----
>>>   1 files changed, 4 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>>> index 932c318..976d332 100644
>>> --- a/kernel/cgroup.c
>>> +++ b/kernel/cgroup.c
>>> @@ -869,13 +869,13 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
>>>   		 * agent */
>>>   		synchronize_rcu();
>>>
>>> -		mutex_lock(&cgroup_mutex);
>>>   		/*
>>>   		 * Release the subsystem state objects.
>>>   		 */
>>>   		for_each_subsys(cgrp->root, ss)
>>>   			ss->destroy(cgrp);
>>>
>>> +		mutex_lock(&cgroup_mutex);
>>>   		cgrp->root->number_of_cgroups--;
>>>   		mutex_unlock(&cgroup_mutex);
>>>
>>> @@ -3994,13 +3994,12 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
>>>
>>>    err_destroy:
>>>
>>> +	mutex_unlock(&cgroup_mutex);
>>>   	for_each_subsys(root, ss) {
>>>   		if (cgrp->subsys[ss->subsys_id])
>>>   			ss->destroy(cgrp);
>>>   	}
>>>
>>> -	mutex_unlock(&cgroup_mutex);
>>> -
>>>   	/* Release the reference count that we took on the superblock */
>>>   	deactivate_super(sb);
>>>
>>> @@ -4349,9 +4348,9 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
>>>   		int ret = cgroup_init_idr(ss, css);
>>>   		if (ret) {
>>>   			dummytop->subsys[ss->subsys_id] = NULL;
>>> +			mutex_unlock(&cgroup_mutex);
>>>   			ss->destroy(dummytop);
>>>   			subsys[i] = NULL;
>>> -			mutex_unlock(&cgroup_mutex);
>>>   			return ret;
>>>   		}
>>>   	}
>>> @@ -4447,10 +4446,10 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
>>>   	 * pointer to find their state. note that this also takes care of
>>>   	 * freeing the css_id.
>>>   	 */
>>> +	mutex_unlock(&cgroup_mutex);
>>>   	ss->destroy(dummytop);
>>>   	dummytop->subsys[ss->subsys_id] = NULL;
>>>
>>
>> I'm not fully sure but...dummytop->subsys[] update can be done without locking ?
>>
> I don't see a reason why updates to subsys[] after destruction shouldn't
> be safe. But maybe I am wrong.
> 
> Tejun? Li?
> 

It's safe for dummpytop->subsys[], but it makes the code a bit subtle.

The worst part is, it's not safe to NULLify subsys[i] without cgroup_mutex. It should be
ok to do that before calling ->destroy(), but again the code becomes a bit subtler.