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

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