linux-kernel - Re: [PATCH 1/2] workqueue: pin the pool while it is managing

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Date:   Thu, 28 May 2020 10:08:44 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH 1/2] workqueue: pin the pool while it is managing

On 2020-05-28 03:06:55 [+0000], Lai Jiangshan wrote:
> So that put_unbound_pool() can ensure all workers in idle,
> no unfinished manager. And it doens't need to wait any manager
> and can go to delete all the idle workers straight away.
> 
> Also removes manager waitqueue, because it is unneeded and as
> Sebastian Andrzej Siewior said:
> 
>   The workqueue code has it's internal spinlock (pool::lock) and also
>   implicit spinlock usage in the wq_manager waitqueue. These spinlocks
>   are converted to 'sleeping' spinlocks on a RT-kernel.
> 
>   Workqueue functions can be invoked from contexts which are truly atomic
>   even on a PREEMPT_RT enabled kernel. Taking sleeping locks from such
>   contexts is forbidden.
> 
>   pool::lock can be converted to a raw spinlock as the lock held times
>   are short. But the workqueue manager waitqueue is handled inside of
>   pool::lock held regions which again violates the lock nesting rules
>   of raw and regular spinlocks.

This seems to work for my test case I had test my chance. And lockdep
didn't complain so…

If you prefer this over my 1/2 what do we do about 2/2? Do you want me
to repost it?

Sebastian