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Message-ID: <6599ad830707170849v11fe8cecs6d172cd38d247e09@mail.gmail.com>
Date: Tue, 17 Jul 2007 08:49:51 -0700
From: "Paul (宝瑠) Menage"
<menage@...gle.com>
To: balbir@...ux.vnet.ibm.com
Cc: dhaval@...ux.vnet.ibm.com, "Pavel Emelianov" <xemul@...ru>,
"linux kernel mailing list" <linux-kernel@...r.kernel.org>,
"Paul Jackson" <pj@....com>,
"Linux Containers" <containers@...ts.osdl.org>,
"Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: Containers: css_put() dilemma
On 7/17/07, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> Paul (??) Menage wrote:
> > On 7/17/07, Balbir Singh <balbir@...ux.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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