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

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Message-ID: <CAJhGHyBmcY75Rhc7UFyK7Ozho+aqOcX2EaxePhZFu9rt0w3-mA@mail.gmail.com>
Date:   Thu, 2 Apr 2020 08:07:35 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Wed, Apr 1, 2020 at 9:03 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> 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()

If it is preempted on *0, the preempting caller drop early in
wq_worker_sleeping()
so there is no atomic decreasing, only one atomic_inc() in the
preempting caller.
The preempted point here, wq_worker_running(), has already just done
atomic_inc(),
the total number of atomic_inc() is two, while the number of atomic decreasing
is one.

>
> - after *1, the preempting task will invoke wq_worker_sleeping() and do
>   dec() + inc().
>
> What did I miss here?
>
> Sebastian