Re: [PATCH 2/2] cgroup: Use separate work structs on css release path

[Tadeusz Struk]

On 5/26/22 11:15, Tejun Heo wrote:
> Hello, Michal.
>
> On Thu, May 26, 2022 at 11:56:34AM +0200, Michal Koutný wrote:
> > // ref=A: initial state
> > kill_css()
> >    css_get // ref+=F == A+F: fuse
> >    percpu_ref_kill_and_confirm
> >      __percpu_ref_switch_to_atomic
> >        percpu_ref_get
> >          // ref += 1 == A+F+1: atomic mode, self-protection
> >      percpu_ref_put
> >        // ref -= 1 == A+F: kill the base reference
> >    [via rcu]
> >    percpu_ref_switch_to_atomic_rcu
> >      percpu_ref_call_confirm_rcu
> >        css_killed_ref_fn == refcnt.confirm_switch
> >          queue_work(css->destroy_work)        (1)
> >                                                       [via css->destroy_work]
> >                                                       css_killed_work_fn == wq.func
> >                                                         offline_css() // needs fuse
> >                                                         css_put // ref -= F == A: de-fuse
> >        percpu_ref_put
> >          // ref -= 1 == A-1: remove self-protection
> >          css_release                                   // A <= 1 -> 2nd queue_work explodes!
>
> I'm not sure I'm following it but it's perfectly fine to re-use the work
> item at this point. The work item actually can be re-cycled from the very
> beginning of the work function. The only thing we need to make sure is that
> we don't css_put() prematurely to avoid it being freed while we're using it.

Yes, it is ok to reuse a work struct, but it's not ok to have the same
work struct enqueued twice on the same WQ when list debug is enabled.
That's why we are getting this "BUG: corrupted list.."

> For the sharing to be a problem, we should be queueing the release work item
> while the destroy instance is still pending, and if that is the case, it
> doesn't really matter whether we use two separate work items or not. We're
> already broken and would just be shifting the problem to explode elsewhere.
>
> The only possibility that I can think of is that somehow we're ending up
> with an extra css_put() somewhere thus triggering the release path
> prematurely. If that's the case, we'll prolly need to trace get/puts to find
> out who's causing the ref imbalance.

That's right. Michal was on the right track for the kill_css() part.
What I think is going on is that once css_create() fails then
cgroup_subtree_control_write() ends up calling first kill_css() and
then css_put() on the same css, I think it's &cgrp->self of the kernfs_node.
The each_live_descendant_post() also iterates on the root.
Here is the call flow (sorry for long lines):

cgroup_subtree_control_write(of)->cgroup_apply_control(cgrp)->cgroup_apply_control_enable(cgrp)->css_create() <- fails here and returns error
  |
  |-> cgroup_finalize_control(cgrp)->cgroup_apply_control_disable(cgrp)->each_live_descendant_post(cgrp)->kill_css()->percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn) <- this triggers css_killed_ref_fn() to be called
  |
  |  css_killed_ref_fn() <- first css->destroy_work enqueue
  |    |
  |    |->  INIT_WORK(&css->destroy_work, css_killed_work_fn); queue_work(cgroup_destroy_wq, &css->destroy_work);
  |
  |
  |-> goto out_unlock;
  |     |
  |     |-> cgroup_kn_unlock(kernfs_node)->cgroup_put(cgrp)->css_put(&cgrp->self)->percpu_ref_put(&css->refcnt) <- this triggers css_release() to be called
  |
  |
     css_release(percpu_ref) <- second css->destroy_work enqueue
       |
       |->  INIT_WORK(&css->destroy_work, css_release_work_fn); queue_work(cgroup_destroy_wq, &css->destroy_work) <- and it fails here with BUG: corrupted list in insert_work; list_add corruption.


What seems to work for me as the simplest fix is to prevent enqueuing a dying
css in css_release() as below. Please let me know if that makes sense to you.

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 1779ccddb734..5618211487cc 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5210,8 +5210,10 @@ static void css_release(struct percpu_ref *ref)
 	struct cgroup_subsys_state *css =
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
-	INIT_WORK(&css->destroy_work, css_release_work_fn);
-	queue_work(cgroup_destroy_wq, &css->destroy_work);
+	if (!(css->flags & CSS_DYING)) {
+		INIT_WORK(&css->destroy_work, css_release_work_fn);
+		queue_work(cgroup_destroy_wq, &css->destroy_work);
+	}
 }
 
 static void init_and_link_css(struct cgroup_subsys_state *css,

--
Thanks,
Tadeusz