Re: Containers: css_put() dilemma

["Paul (宝瑠) Menage" <menage@google.com>]

On 7/16/07, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> -       if (notify_on_release(cont)) {
> +       if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cont)) {

This seems like a good idea, as long as atomic_dec_and_test() isn't
noticeably more expensive than atomic_dec(). I assume it shouldn't
need to be, since the bus locking operations are presumably the same
in each case.

>                 mutex_lock(&container_mutex);
>                 set_bit(CONT_RELEASABLE, &cont->flags);
> -               if (atomic_dec_and_test(&css->refcnt)) {
> -                       check_for_release(cont);
> -               }
> +               check_for_release(cont);
>                 mutex_unlock(&container_mutex);
>
> That way we set the CONT_RELEASABLE bit only when the ref count drops
> to zero.

That's probably a good idea, in conjunction with another part of my
patch for this that frees container objects under RCU - as soon as you
do the atomic_dec_and_test(), then in theory some other thread could
delete the container (since we're no longer going to be taking
container_mutex in this function). But as long as the container object
remains valid until synchronize_rcu() completes, then we can safely
set the CONT_RELEASABLE bit on it.

> Yes, that is correct, the advantage is that with can_destroy() we
> don't need to go through release synchronization each time we do
> a css_put().

I think the amount of release synchronization *needed* is going to be
the same whether you have the refcounting done in the subsystem or in
the framework. But I agree that right now we're doing one more atomic
op than we strictly need to, and can remove it.

Paul
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