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Date:   Tue, 4 Sep 2018 15:44:50 +0200
From:   Niklas Cassel <niklas.cassel@linaro.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Kevin Shanahan <kevin@shanahan.id.au>,
        Siegfried Metz <frame@mailbox.org>,
        linux-kernel@vger.kernel.org, rafael.j.wysocki@intel.com,
        len.brown@intel.com, rjw@rjwysocki.net, diego.viola@gmail.com,
        rui.zhang@intel.com, viktor_jaegerskuepper@freenet.de
Subject: Re: REGRESSION: boot stalls on several old dual core Intel CPUs
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On Mon, Sep 03, 2018 at 11:33:05AM +0200, Peter Zijlstra wrote:
> On Mon, Sep 03, 2018 at 10:54:23AM +0200, Peter Zijlstra wrote:
> > On Mon, Sep 03, 2018 at 09:38:15AM +0200, Thomas Gleixner wrote:
> > > On Mon, 3 Sep 2018, Peter Zijlstra wrote:
> > > > On Sat, Sep 01, 2018 at 11:51:26AM +0930, Kevin Shanahan wrote:
> > > > > commit 01548f4d3e8e94caf323a4f664eb347fd34a34ab
> > > > > Author: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > > > > Date:   Tue Aug 18 17:09:42 2009 +0200
> > > > > 
> > > > >     clocksource: Avoid clocksource watchdog circular locking dependency
> > > > > 
> > > > >     stop_machine from a multithreaded workqueue is not allowed because
> > > > >     of a circular locking dependency between cpu_down and the workqueue
> > > > >     execution. Use a kernel thread to do the clocksource downgrade.
> > > > 
> > > > I cannot find stop_machine usage there; either it went away or I need to
> > > > like wake up.
> > > 
> > > timekeeping_notify() which is involved in switching clock source uses stomp
> > > machine.
> > 
> > ARGH... OK, lemme see if I can come up with something other than
> > endlessly spawning that kthread.
> > 
> > A special purpose kthread_worker would make more sense than that.
> 
> Can someone test this?
> 
> ---
>  kernel/time/clocksource.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index f74fb00d8064..898976d0082a 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -112,13 +112,28 @@ static int finished_booting;
>  static u64 suspend_start;
>  
>  #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
> -static void clocksource_watchdog_work(struct work_struct *work);
> +static void clocksource_watchdog_work(struct kthread_work *work);
>  static void clocksource_select(void);
>  
>  static LIST_HEAD(watchdog_list);
>  static struct clocksource *watchdog;
>  static struct timer_list watchdog_timer;
> -static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
> +
> +/*
> + * We must use a kthread_worker here, because:
> + *
> + *   clocksource_watchdog_work()
> + *     clocksource_select()
> + *       __clocksource_select()
> + *         timekeeping_notify()
> + *           stop_machine()
> + *
> + * cannot be called from a reqular workqueue, because of deadlocks between
> + * workqueue and stopmachine.
> + */
> +static struct kthread_worker *watchdog_worker;
> +static DEFINE_KTHREAD_WORK(watchdog_work, clocksource_watchdog_work);
> +
>  static DEFINE_SPINLOCK(watchdog_lock);
>  static int watchdog_running;
>  static atomic_t watchdog_reset_pending;
> @@ -158,7 +173,7 @@ static void __clocksource_unstable(struct clocksource *cs)
>  
>  	/* kick clocksource_watchdog_work() */
>  	if (finished_booting)
> -		schedule_work(&watchdog_work);
> +		kthread_queue_work(watchdog_worker, &watchdog_work);
>  }
>  
>  /**
> @@ -199,7 +214,7 @@ static void clocksource_watchdog(struct timer_list *unused)
>  		/* Clocksource already marked unstable? */
>  		if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
>  			if (finished_booting)
> -				schedule_work(&watchdog_work);
> +				kthread_queue_work(watchdog_worker, &watchdog_work);
>  			continue;
>  		}
>  
> @@ -269,7 +284,7 @@ static void clocksource_watchdog(struct timer_list *unused)
>  			 */
>  			if (cs != curr_clocksource) {
>  				cs->flags |= CLOCK_SOURCE_RESELECT;
> -				schedule_work(&watchdog_work);
> +				kthread_queue_work(watchdog_worker, &watchdog_work);
>  			} else {
>  				tick_clock_notify();
>  			}
> @@ -418,7 +433,7 @@ static int __clocksource_watchdog_work(void)
>  	return select;
>  }
>  
> -static void clocksource_watchdog_work(struct work_struct *work)
> +static void clocksource_watchdog_work(struct kthread_work *work)
>  {
>  	mutex_lock(&clocksource_mutex);
>  	if (__clocksource_watchdog_work())
> @@ -806,6 +821,7 @@ static int __init clocksource_done_booting(void)
>  {
>  	mutex_lock(&clocksource_mutex);
>  	curr_clocksource = clocksource_default_clock();
> +	watchdog_worker = kthread_create_worker(0, "cs-watchdog");

Hello Peter,

watchdog_worker is only defined if CONFIG_CLOCKSOURCE_WATCHDOG is set,
so you might want to wrap it with an ifdef to avoid build errors.

Kind regards,
Niklas

>  	finished_booting = 1;
>  	/*
>  	 * Run the watchdog first to eliminate unstable clock sources