Re: [PATCH v2] cgroup: Reorganize css_set_lock and kernfs path processing

[Tejun Heo]

Hello,

On Wed, Sep 28, 2022 at 01:33:16PM +0200, Michal Koutný wrote:
...
> I realized the pinning with reference taking won't really work
> generally. The code would get the reference within RCU read section, so
> it'd have to be cgroup_get_live() and if that fails there's not much to
> do.

Hmm... isn't current's root cgrp guaranteed to be alive? How would
cgroup_get_live() fail? Also, shouldn't cgroup_get() enough for path
walking?

> @@ -6673,8 +6678,8 @@ struct cgroup *cgroup_get_from_path(const char *path)
>  
>  	spin_lock_irq(&css_set_lock);
>  	root_cgrp = current_cgns_cgroup_from_root(&cgrp_dfl_root);
> -	kn = kernfs_walk_and_get(root_cgrp->kn, path);
>  	spin_unlock_irq(&css_set_lock);
> +	kn = kernfs_walk_and_get(root_cgrp->kn, path);

If you really wanna do it this way, can you please add a detailed comment
here why this is safe? But I'd prefer just doing a strightforward ref
inc/dec around it.

Thanks.

-- 
tejun