Containers: css_put() dilemma

Balbir Singh balbir at
Tue Jul 17 10:40:59 PDT 2007

Paul (??) Menage wrote:
> Because as soon as you do the atomic_dec_and_test() on css->refcnt and
> the refcnt hits zero, then theoretically someone other thread (that
> already holds container_mutex) could check that the refcount is zero
> and free the container structure.

Hi, Paul,

That sounds correct. I wonder now if the solution should be some form
of delegation for deletion of unreferenced containers (HINT: work queue
or kernel threads).

> Adding a synchronize_rcu in container_diput() guarantees that the
> container structure won't be freed while someone may still be
> accessing it.

Do we take rcu_read_lock() in css_put() path or use call_rcu() to
free the container?

>> Could you please elaborate as to why using a release agent is broken
>> when the memory controller is attached to it?
> Because then it will try to take container_mutex in css_put() if it
> drops the last reference to a container, which is the thing that you
> said you had to avoid since you called css_put() in contexts that
> couldn't sleep.
> Paul

	Warm Regards,
	Balbir Singh
	Linux Technology Center

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