Hello, Vladimir.
On Fri, Dec 06, 2013 at 11:02:07AM +0400, Vladimir Davydov wrote:
> This patch fixes this bug, but I have a couple of questions regarding it.
>
> First, cgroup_load_subsys() also calls css_online(), and if it fails, it
> calls cgroup_unload_subsys() to rollback. The latter function executes
> the following command:
>
> offline_css(cgroup_css(cgroup_dummy_top, ss));
>
> But since we failed to online_css(), cgroup_css() will return NULL
> resulting in another oops.
I don't think the root css onlining fails for any existing controllers
but yeah that looks wrong. Can you please send a patch?
> Second, it's not clear to me why we need the CSS_ONLINE flag at all if
> we never assign css's that we fail to online to a cgroup. AFAIU we will
> never see such css's, because in all places we call offline_css(),
> namely cgroup_destroy_locked() (via kill_css()) and
> cgroup_unload_subsys(), we use cgroup_css() which will return NULL for them.
The whole thing is in flux and will look very different in near
future. I actually had patches queued which deal with the issue you
spotted but they are being blocked on other changes ATM. So, yeah,
there are some spurious stuff now.
> Before we get here, we call
>
> /* each css holds a ref to the cgroup's dentry and the parent css */
> for_each_root_subsys(root, ss) {
> struct cgroup_subsys_state *css = css_ar[ss->subsys_id];
>
> dget(dentry);
> css_get(css->parent);
> }
>
> If we fail to online a css, we will only call
>
> ss->css_free(css);
>
> on it skipping css_put() on parent.
>
> css_put() is called on parent in css_release() on normal destroy path.
You're right. I'll revise the patch.
Thanks.
--
tejun
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