linux-kernel - Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER

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Date:   Sat, 16 Jan 2021 16:16:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

On Sat, Jan 16, 2021 at 10:45:04PM +0800, Lai Jiangshan wrote:
> On Sat, Jan 16, 2021 at 8:45 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > It is also the exact sequence normal per-cpu threads (smpboot) use to
> > preserve affinity.
> 
> Other per-cpu threads normally do short-live works. wq's work can be
> lengthy, cpu-intensive, heavy-lock-acquiring or even call
> get_online_cpus() which might result in a deadlock with kthread_park().

kthread_park() is called by the migration thread running the
workqueue_online_cpu() callback.

kthread_parkme() is called by the worker thread, after it completes a
work and has no locks held from that context.