[PATCH] cgroup: Fix root_count when mount fails due to busy subsystem

[Serge E. Hallyn]

Quoting Paul Menage (menage at google.com):
> cgroup: Fix root_count when mount fails due to busy subsystem
> 
> root_count was being incremented in cgroup_get_sb() after all error
> checking was complete, but decremented in cgroup_kill_sb(), which can
> be called on a superblock that we gave up on due to an error.  This
> patch changes cgroup_kill_sb() to only decrement root_count if the
> root was previously linked into the list of roots.
> 
> Signed-off-by: Paul Menage <menage at google.com>

Tested-by: Serge Hallyn <serue at us.ibm.com>

thanks,
-serge

> 
> ---
> 
> I was actually surprised to find that list_del() doesn't crash when
> run on an unattached list_head structure.
> 
>  kernel/cgroup.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index adcd0bb..9ce27e8 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1115,8 +1115,10 @@ static void cgroup_kill_sb(struct super_block *sb) {
>  	}
>  	write_unlock(&css_set_lock);
> 
> -	list_del(&root->root_list);
> -	root_count--;
> +	if (!list_empty(&root->root_list)) {
> +		list_del(&root->root_list);
> +		root_count--;
> +	}
> 
>  	mutex_unlock(&cgroup_mutex);
>