Re: [PATCH] cgroup: don't queue css_release_work if one already pending

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

On 5/20/22 01:13, Tejun Heo wrote:
> On Thu, May 19, 2022 at 04:26:51PM -0700, Tadeusz Struk wrote:
>> On 5/19/22 04:23, Hillf Danton wrote:
>>> On Wed, 18 May 2022 09:48:21 -0700 Tadeusz Struk  wrote:
>>>> On 4/22/22 04:05, Michal Koutny wrote:
>>>>> On Thu, Apr 21, 2022 at 02:00:56PM -1000, Tejun Heo<tj@kernel.org>  wrote:
>>>>>> If this is the case, we need to hold an extra reference to be put by the
>>>>>> css_killed_work_fn(), right?
>>> That put could trigger INIT_WORK in css_release() and warning [1]
>>> on init active (active state 0) object OTOH as the same
>>> css->destroy_work is used in both kill and release paths.
> 
> Hmm... wouldn't the extra reference keep release from happening?
> 
>> Will this help if there would be two WQs, one for the css_release path
>> and one for the rcu_work?
>>
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index adb820e98f24..a4873b33e488 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -124,6 +124,7 @@ DEFINE_PERCPU_RWSEM(cgroup_threadgroup_rwsem);
>>    * which may lead to deadlock.
>>    */
>>   static struct workqueue_struct *cgroup_destroy_wq;
>> +static struct workqueue_struct *cgroup_destroy_rcu_wq;
> 
> I don't understand why this would help. Care to elaborate?

I think it will help to solve the list corruption issue:

list_add corruption. prev->next should be next (ffff8881f705c060), but was ffff888113123870. (prev=ffff888113123870).
------------[ cut here ]------------
kernel BUG at lib/list_debug.c:28!

as this is a result of enqueuing the same css->destroy_work onto the same WQ,
one on the rcu path and one on the css_release path.
I will prototype it today and test with syzbot.

-- 
Thanks,
Tadeusz