Re: [PATCH 1/4] Support named cgroups hierarchies

[Li Zefan <lizf@cn.fujitsu.com>]

>>>  static int cgroup_set_super(struct super_block *sb, void *data)
>>>  {
>>>       int ret;
>>> -     struct cgroupfs_root *root = data;
>>> +     struct cgroup_sb_opts *opts = data;
>>> +
>>> +     /* If we don't have a new root, we can't set up a new sb */
>>> +     if (!opts->new_root)
>>> +             return -EINVAL;
>>> +
>> I think this should be BUG_ON(). If set_super() is called,
>> we are allocating a new root, so opts->new_root won't be NULL.
> 
> Not true - if you try to mount a hierarchy by name, but with no
> subsystem options, then we don't construct a new root, but we still
> call sget(). If we find a superblock with the right name then we use
> it, else sget() will allocate a new superblock and call
> cgroup_set_super(), at which point we need to fail.
> 

Ah, I see.

>>> +             struct cgroupfs_root *new_root = cgroup_root_from_opts(&opts);
>> Why not just declare new_root in the beginning of cgroup_get_sb()?
> 
> Because it's not needed for the entire scope of the function. Keeping
> its scope as small as possible makes it clearer what it's being used
> for.
> 

If we had been doing this, we'll see many:

	(no if, while, for)
	{
		...
	}

in kernel code, but I don't remember I ever saw this style.