linux-kernel - Re: [RFC] [PATCH 1/5] cgroups: revamp subsys array

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Date:	Tue, 08 Dec 2009 15:38:43 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Ben Blum <bblum@...rew.cmu.edu>
CC:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	menage@...gle.com
Subject: Re: [RFC] [PATCH 1/5] cgroups: revamp subsys array

> @@ -1291,6 +1324,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
>  	struct cgroupfs_root *new_root;
>  
>  	/* First find the desired set of subsystems */
> +	down_read(&subsys_mutex);

Hmm.. this can lead to deadlock. sget() returns success with sb->s_umount
held, so here we have:

	down_read(&subsys_mutex);

	down_write(&sb->s_umount);

On the other hand, sb->s_umount is held before calling kill_sb(),
so when umounting we have:

	down_write(&sb->s_umount);

	down_read(&subsys_mutex);

>  	ret = parse_cgroupfs_options(data, &opts);
>  	if (ret)
>  		goto out_err;
> @@ -1367,6 +1401,8 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
>  			free_cg_links(&tmp_cg_links);
>  			goto drop_new_super;
>  		}
> +		/* done with subsys stuff and no other failure case */
> +		up_read(&subsys_mutex);
>  
>  		/* EBUSY should be the only error here */
>  		BUG_ON(ret);
> @@ -1415,6 +1451,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
>   drop_new_super:
>  	deactivate_locked_super(sb);
>   out_err:
> +	up_read(&subsys_mutex);
>  	kfree(opts.release_agent);
>  	kfree(opts.name);
>  
> @@ -1434,10 +1471,12 @@ static void cgroup_kill_sb(struct super_block *sb) {
>  	BUG_ON(!list_empty(&cgrp->children));
>  	BUG_ON(!list_empty(&cgrp->sibling));
>  
> +	down_read(&subsys_mutex);
>  	mutex_lock(&cgroup_mutex);
>  
>  	/* Rebind all subsystems back to the default hierarchy */
>  	ret = rebind_subsystems(root, 0);
> +	up_read(&subsys_mutex);
>  	/* Shouldn't be able to fail ... */
>  	BUG_ON(ret);
>  


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