linux-kernel - Re: Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous

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Message-ID: <mhwbjfm3zsoinm2eozz6d7djtflf6tbpmfzsbbsscv3u5aijtr@p2edxl6voxoa>
Date: Wed, 3 Apr 2024 16:54:20 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>, 
	Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>, 
	Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	linux-pm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Frederic Weisbecker <frederic@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Ingo Molnar <mingo@...nel.org>, Anna-Maria Behnsen <anna-maria@...utronix.de>, 
	Alex Shi <alexs@...nel.org>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing
 synchronous

On Wed, Apr 03, 2024 at 04:26:38PM +0200, Valentin Schneider <vschneid@...hat.com> wrote:
> Also, I gave Michal's patch a try and it looks like it's introducing a

Thank you.

>   cgroup_threadgroup_rwsem -> cpuset_mutex
> ordering from
>   cgroup_transfer_tasks_locked()
>   `\
>     percpu_down_write(&cgroup_threadgroup_rwsem);
>     cgroup_migrate()
>     `\
>       cgroup_migrate_execute()
>       `\
>         ss->can_attach() // cpuset_can_attach()
>         `\
>           mutex_lock(&cpuset_mutex);
> 
> which is invalid, see below.

_This_ should be the right order (cpuset_mutex inside
cgroup_threadgroup_rwsem), at least in my mental model. Thus I missed
that cpuset_mutex must have been taken somewhere higher up in the
hotplug code (CPU 0 in the lockdep dump, I can't easily see where from)
:-/.

Michal

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