Re: [RFC][PATCH 8/9 v2] cgroup: avoid creating new cgroup under a cgroup being destroyed

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Fri, Apr 27, 2012 at 03:04:14PM +0900, KAMEZAWA Hiroyuki wrote:
> When ->pre_destroy() is called, it should be guaranteed that
> new child cgroup is not created under a cgroup, where pre_destroy()
> is running. If not, ->pre_destroy() must check children and
> return -EBUSY, which causes warning.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>

Hmm... I'm getting confused more.  Why do we need these cgroup changes
at all?  cgroup still has cgrp->count check and
cgroup_clear_css_refs() after pre_destroy() calls.  The order of
changes should be,

* Make memcg pre_destroy() not fail; however, pre_destroy() should
  still be ready to be retried.  That's the defined interface.

* cgroup core updated to drop pre_destroy() retrying and guarantee
  that pre_destroy() invocation will happen only once.

* memcg and other cgroups can update their pre_destroy() if the "won't
  be retried" part can simplify their implementations.

So, there's no reason to be updating cgroup pre_destroy() semantics at
this point and these updates actually break cgroup API as it currently
stands.  The only change necessary is memcg's pre_destroy() not
returning zero.

Thanks.

-- 
tejun