linux-kernel - Re: [PATCH] cgroup: don't queue css_release

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Message-ID: <YodNu2C5iHKW3UeZ@slm.duckdns.org>
Date:   Thu, 19 May 2022 22:13:47 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Tadeusz Struk <tadeusz.struk@...aro.org>
Cc:     Hillf Danton <hdanton@...a.com>, Michal Koutny <mkoutny@...e.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzbot+e42ae441c3b10acf9e9d@...kaller.appspotmail.com
Subject: Re: [PATCH] cgroup: don't queue css_release_work if one already
 pending

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@...nel.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 pathes.

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?

Thanks.

-- 
tejun