linux-kernel - Re: [PATCH 1/1] kthread: break dependency between worker->lock and task_struct->pi

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Date:   Tue, 28 Apr 2020 18:31:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     mingo@...hat.com, hannes@...xchg.org, will@...nel.org,
        akpm@...ux-foundation.org, tglx@...utronix.de,
        ben.dooks@...ethink.co.uk, cl@...k-chips.com, ke.wang@...soc.com,
        shakeelb@...gle.com, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 1/1] kthread: break dependency between worker->lock and
 task_struct->pi_lock

On Mon, Apr 27, 2020 at 11:43:58AM -0700, Suren Baghdasaryan wrote:
> A number of kthread-related functions indirectly take task_struct->pi_lock
> while holding worker->lock in the call chain like this:
>     spin_lock(&worker->lock)
>     kthread_insert_work
>     wake_up_process
>     try_to_wake_up
>     raw_spin_lock_irqsave(&p->pi_lock, flags)
> 
> This lock dependency exists whenever kthread_insert_work is called either
> directly or indirectly via __kthread_queue_delayed_work in the following
> functions:
>     kthread_queue_work
>     kthread_delayed_work_timer_fn
>     kthread_queue_delayed_work
>     kthread_flush_work
>     kthread_mod_delayed_work
> 
> This creates possibilities for circular dependencies like the one reported
> at: https://lkml.org/lkml/2020/4/24/954

Please, do not use lkml.org links.

Also, ideally, we'd pull that kthread_queue_delayed_work() out from
under rq->lock.

In fact, looking at it, WTH is the delayed branch of
kthread_queue_delayed_work() under that lock? That whole
delayed_work_list thing smells like bong-hits.