linux-kernel - Re: [PATCH] workqueue: Don't double assign worker->sleeping

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Date:   Wed, 1 Apr 2020 15:03:46 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     kernel test robot <lkp@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...ts.01.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] workqueue: Don't double assign worker->sleeping

On 2020-04-01 11:44:06 [+0800], Lai Jiangshan wrote:
> On Wed, Apr 1, 2020 at 11:22 AM Lai Jiangshan <jiangshanlai@...il.com> wrote:
> >
> > Hello
Hi Lai,

…
> > 2) wq_worker_running() can be interrupted(async-page-faulted in virtual machine)
> > and nr_running would be decreased twice.
> 
> would be *increased* twice
> 
> I just saw the V2 patch, this issue is not listed, but need to be fixed too.

| void wq_worker_running(struct task_struct *task)
| {
|         struct worker *worker = kthread_data(task);
| 
|         if (!worker->sleeping)
|                 return;
|         if (!(worker->flags & WORKER_NOT_RUNNING))
|                 atomic_inc(&worker->pool->nr_running);
*0
|         worker->sleeping = 0;
*1
| }

So an interrupt
- before *0, the preempting caller drop early in wq_worker_sleeping(), only one
  atomic_inc()

- after *1, the preempting task will invoke wq_worker_sleeping() and do
  dec() + inc().

What did I miss here?

Sebastian