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SubjectRe: [PATCH 1/3] cgroup: fix pre_destroy and semantics of css->refcnt
On Wed, 03 Dec 2008 11:44:36 +0800
Li Zefan <lizf@cn.fujitsu.com> wrote:

> > +/*
> > + * Try to set all subsys's refcnt to be 0.
> > + * css->refcnt==0 means this subsys will be destroy()'d.
> > + */
> > +static bool cgroup_set_subsys_removed(struct cgroup *cgrp)
> > +{
> > + struct cgroup_subsys *ss;
> > + struct cgroup_subsys_state *css, *tmp;
> > +
> > + for_each_subsys(cgrp->root, ss) {
> > + css = cgrp->subsys[ss->subsys_id];
> > + if (!atomic_dec_and_test(&css->refcnt))
> > + goto rollback;
> > + }
> > + return true;
> > +rollback:
> > + for_each_subsys(cgrp->root, ss) {
> > + tmp = cgrp->subsys[ss->subsys_id];
> > + atomic_inc(&tmp->refcnt);
> > + if (tmp == css)
> > + break;
> > + }
> > + return false;
> > +}
> > +
>
> This function may return false, then causes rmdir() fail. So css_tryget(subsys1)
> returns 0 doesn't necessarily mean subsys1->destroy() will be called,
> if subsys2's css's refcnt is >1 when cgroup_set_subsys_removed() is called.
>
> Will this bring up bugs and problems?
>

current user of css_get() is only memcg, so no problem now.

"css_tryget() fails" means "rmdir" is called against this cgroup. So, not so
troublesome in genral, I think. (the user will retry rmdir()).

To be honest, I don't want to return -EBUSY but wait for success in the kernel.
and go back to pre_destroy() for this temporal race.

Thanks,
-Kame







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