linux-kernel - Re: [PATCH] workqueue: Remove rcu_read_lock/unlock() in workqueue

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Message-ID: <20210218151732.GR2743@paulmck-ThinkPad-P72>
Date:   Thu, 18 Feb 2021 07:17:32 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     "Zhang, Qiang" <qiang.zhang@...driver.com>,
        Tejun Heo <tj@...nel.org>, Tejun Heo <htejun@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] workqueue: Remove rcu_read_lock/unlock() in
 workqueue_congested()

On Thu, Feb 18, 2021 at 11:04:00AM +0800, Lai Jiangshan wrote:
> +CC Paul
> 
> 
> On Wed, Feb 17, 2021 at 7:58 PM <qiang.zhang@...driver.com> wrote:
> >
> > From: Zqiang <qiang.zhang@...driver.com>
> >
> > The RCU read critical area already by preempt_disable/enable()
> > (equivalent to rcu_read_lock_sched/unlock_sched()) mark, so remove
> > rcu_read_lock/unlock().
> 
> I think we can leave it which acks like document, especially
> workqueue_congested() is not performance crucial.  Either way
> is Ok for me.

If the rcu_read_lock() is removed, should there be a comment saying that
it interacts with synchronize_rcu()?  Just in case one of the real-time
guys figures out a way to get the job done without disabling preemption...

							Thanx, Paul

> If it needs to be changed, please also do the same for
> rcu_read_lock() in wq_watchdog_timer_fn().
> 
> And __queue_work() and try_to_grab_pending() also use local_irq_save()
> and rcu_read_lock() at the same time, but I don't know will these
> local_irq_save() be changed to raw_local_irq_save() in PREEMPT_RT.
> 
> 
> >
> > Signed-off-by: Zqiang <qiang.zhang@...driver.com>
> > ---
> >  kernel/workqueue.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 0d150da252e8..c599835ad6c3 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -4540,7 +4540,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
> >         struct pool_workqueue *pwq;
> >         bool ret;
> >
> > -       rcu_read_lock();
> >         preempt_disable();
> >
> >         if (cpu == WORK_CPU_UNBOUND)
> > @@ -4553,7 +4552,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
> >
> >         ret = !list_empty(&pwq->delayed_works);
> >         preempt_enable();
> > -       rcu_read_unlock();
> >
> >         return ret;
> >  }
> > --
> > 2.25.1
> >