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

From: Glauber Costa <[email-masked]>
Date: Tue, 24 Apr 2012 08:42:32 -0300

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?