Containers: css_put() dilemma

Paul (宝瑠) Menage menage at google.com
Tue Jul 17 08:49:51 PDT 2007


On 7/17/07, Balbir Singh <balbir at linux.vnet.ibm.com> wrote:
> Paul (??) Menage wrote:
> > On 7/17/07, Balbir Singh <balbir at linux.vnet.ibm.com> wrote:
> >> >
> >> > >                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);
> >> > >
> >
> > I think that this isn't safe as it stands, without a synchronize_rcu()
> > in container_diput() prior to the kfree(). Also, it will break if
> > anyone tries to use a release agent on a hierarchy that has your
> > memory controller bound to it.
> >
>
>
> Isn't the code functionally the same as before? We still do atomic_test_and_dec()
> as before. We still set_bit() CONT_RELEASABLE, we take the container_mutex
> and check_for_release(). I am not sure I understand what changed?

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.

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

>
> 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


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