Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230523140942.18679-1-qiang.zhang1211@gmail.com>
 <ZG0WoOqfr6R5wK1p@slm.duckdns.org> <ZG0YeeTZNox8YB25@slm.duckdns.org>
In-Reply-To: <ZG0YeeTZNox8YB25@slm.duckdns.org>
From:   Z qiang <qiang.zhang1211@gmail.com>
Date:   Wed, 24 May 2023 11:39:29 +0800
Message-ID: <CALm+0cX7URDtGJfeE_BBTtxLhw34JQwsLcXUC9RBmnTsG23SOw@mail.gmail.com>
Subject: Re: [PATCH v2] workqueue: Fix warning triggered when nr_running is
 checked in worker_enter_idle()
To:     Tejun Heo <tj@kernel.org>
Cc:     jiangshanlai@gmail.com, naresh.kamboju@linaro.org,
        linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

>
> Hello,
>
> On Tue, May 23, 2023 at 09:40:16AM -1000, Tejun Heo wrote:
> > On Tue, May 23, 2023 at 10:09:41PM +0800, Zqiang wrote:
> > > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > > index 9c5c1cfa478f..329b84c42062 100644
> > > --- a/kernel/workqueue.c
> > > +++ b/kernel/workqueue.c
> > > @@ -1144,13 +1144,12 @@ void wq_worker_tick(struct task_struct *task)
> > >      * longer than wq_cpu_intensive_thresh_us, it's automatically marked
> > >      * CPU_INTENSIVE to avoid stalling other concurrency-managed work items.
> > >      */
> > > -   if ((worker->flags & WORKER_NOT_RUNNING) ||
> > > +   if ((worker->flags & WORKER_NOT_RUNNING) || worker->sleeping ||
> > >         worker->task->se.sum_exec_runtime - worker->current_at <
> > >         wq_cpu_intensive_thresh_us * NSEC_PER_USEC)
> > >             return;
> >
> > Ah, right, this isn't just interrupted read-modify-write. It has to consider
> > sleeping. This is subtle. We'll definitely need more comments. Will think
> > more about it.
>
> So, there already are enough barriers to make this safe but that's kinda
> brittle because e.g. it'd depend on the barrier in preempt_disable() which
> is there for an unrelated reason. Can you please change ->sleeping accesses
> to use WRITE/READ_ONCE() and explain in wq_worker_tick() that the worker
> doesn't contribute to ->nr_running while ->sleeping regardless of
> NOT_RUNNING and thus the operation shouldn't proceed? We probably need to
> make it prettier but I think that should do for now.

Thanks for the suggestion, I will resend.

>
> Thanks.
>
> --
> tejun