linux-kernel - Re: [PATCH -v2] cgroup: fix deadlock caused by cgroup_mutex and cpu_hotplug

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Message-ID: <ohqau62jzer57mypyoiic4zwhz2zxwk5rsni4softabxyybgke@nnsqdj2dbvkl>
Date: Fri, 26 Jul 2024 15:04:24 +0200
From: Michal Koutný <mkoutny@...e.com>
To: chenridong <chenridong@...wei.com>
Cc: Hillf Danton <hdanton@...a.com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, tj@...nel.org, bpf@...r.kernel.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -v2] cgroup: fix deadlock caused by cgroup_mutex and
 cpu_hotplug_lock

Hello.

On Thu, Jul 25, 2024 at 09:48:36AM GMT, chenridong <chenridong@...wei.com> wrote:
> > > This issue can be reproduced by the following methods:
> > > 1. A large number of cpuset cgroups are deleted.
> > > 2. Set cpu on and off repeatly.
> > > 3. Set watchdog_thresh repeatly.

BTW I assume this is some stress testing, not a regular use scenario of
yours, right?

> > > 
> > > The reason for this issue is cgroup_mutex and cpu_hotplug_lock are
> > > acquired in different tasks, which may lead to deadlock.
> > > It can lead to a deadlock through the following steps:
> > > 1. A large number of cgroups are deleted, which will put a large
> > >     number of cgroup_bpf_release works into system_wq. The max_active
> > >     of system_wq is WQ_DFL_ACTIVE(256). When cgroup_bpf_release can not
> > >     get cgroup_metux, it may cram system_wq, and it will block work
> > >     enqueued later.

Who'd be the holder of cgroup_mutex preventing cgroup_bpf_release from
progress? (That's not clear to me from your diagram.)

...
> > Given idle worker created independent of WQ_DFL_ACTIVE before handling
> > work item, no deadlock could rise in your scenario above.
> 
> Hello Hillf, did you mean to say this issue couldn't happen?

Ridong, can you reproduce this with CONFIG_PROVE_LOCKING (or do you have
lockdep message from it aready)? It'd be helpful to get insight into
the suspected dependencies.

Thanks,
Michal

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