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

From: KAMEZAWA Hiroyuki <[email-masked]>
Date: Tue, 24 Apr 2012 11:31:54 +0900

(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 ?

Thanks,
-Kame