Re: [RFC][PATCH 1/4] New css->refcnt implementation.

From: KAMEZAWA Hiroyuki
Date: Fri Dec 05 2008 - 06:24:21 EST

Thank you for comments.

Paul Menage said:
> On Fri, Dec 5, 2008 at 12:28 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
>> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxx>
>> Now, the last check of refcnt is done after pre_destroy(), so rmdir()
>> can fail
>> after pre_destroy(). But memcg set mem->obsolete to be 1 at pre_destroy.
>> This is a bug. So, removing memcg->obsolete flag is sane.
>> But there is no interface to confirm "css" is oboslete or not. I.e.
>> there is
>> no flag to check whether we can increase css_refcnt or not!
> The basic rule is that you're only supposed to increment the css
> refcount if you have:
> - a reference to a task in the cgroup (that is pinned via task_lock()
> so it can't be moved away)
> or
> - an existing reference to the css
My problem is that we can do css_get() after pre_destroy() and
css's refcnt goes down to 0.

>> This patch changes this css->refcnt rule as following
>> - css->refcnt is no longer private counter, just point to
>> css->cgroup->css_refcnt.
> The reason I didn't do this is that I'd like to keep the ref counts
> separate to make it possible to add/remove subsystems from a hiearchy
> - if they're all mingled into a single refcount, it's impossible to
> tell if a particular subsystem has refcounts.
It's not problem. please see memcg, it has its own refcnt.
and memcg subsystem is not destroyed at destroy(), now.

What I want to is atomic check to

"Can I access css->group ?"

I once tried to do
if (cgroup_is_removed(&memcg->css.cgroup)) {
return 0;
But this seems not to work.

>> - css_put() is changed not to call notify_on_release().
>> From documentation, notify_on_release() is called when there is
>> no
>> tasks/children in cgroup. On implementation, notify_on_release
>> is
>> not called if css->refcnt > 0.
> The documentation is a little inaccurate - it's called when the cgroup
> is removable. In the original cpusets this implied that there were no
> tasks or children; in cgroups, a refcount can keep the group alive
> too, so it's right to not call notify_on_release if there are
> remaining refcounts.

>> This is problem. Memcg has css->refcnt by each page even when
>> there are no tasks. Release handler will be never called.
> Right, because it can't remove the dir if there are still refcounts.
> Early in the development of cgroups I did have a css refcount scheme
> similar to what you have, with tryget, etc, but still with separate
> refcounts for each subsystem; I got rid of it since it seemed more
> complicated than we needed at the time. But I'll see if I can dig it
> up.

I'll not do extra work on this "notify handler" for a while.
But please, it's now broken.

Hmm...but removing memcg->obsolete flag is difficult. Because
I can't guarantee memcg->css.cgroup is valid pointer.

Can I add a flag CSS_REMOVED ? (set after CGROUP_REMOVED flag) ?
Then, I'll be able to have some work around.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at