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

[Tadeusz Struk <tadeusz.struk@linaro.org>]

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