linux-kernel - Re: [PATCH 3/3] clocksource: fix can not set tsc as clocksource bug

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Date:	Fri, 5 Jul 2013 07:41:09 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alex Shi <alex.shi@...el.com>
cc:	hpa@...ux.intel.com, tim.c.chen@...ux.intel.com,
	linux-kernel@...r.kernel.org, andi.kleen@...el.com,
	a.p.zijlstra@...llo.nl, mingo@...e.hu
Subject: Re: [PATCH 3/3] clocksource: fix can not set tsc as clocksource
 bug

On Fri, 5 Jul 2013, Alex Shi wrote:
> On 07/04/2013 09:04 PM, Thomas Gleixner wrote:
> >>> > >  static int clocksource_watchdog_kthread(void *data)
> >>> > >  {
> >>> > >  	struct clocksource *cs, *tmp;
> >>> > > @@ -412,11 +415,14 @@ static int clocksource_watchdog_kthread(void *data)
> >>> > >  
> >>> > >  	mutex_lock(&clocksource_mutex);
> >>> > >  	spin_lock_irqsave(&watchdog_lock, flags);
> >>> > > -	list_for_each_entry_safe(cs, tmp, &watchdog_list, wd_list)
> >>> > > +	list_for_each_entry_safe(cs, tmp, &watchdog_list, wd_list) {
> >>> > >  		if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
> >>> > >  			list_del_init(&cs->wd_list);
> >>> > >  			list_add(&cs->wd_list, &unstable);
> >>> > >  		}
> >>> > > +		if (cs->flags & CLOCK_SOURCE_VALID_FOR_HRES)
> >>> > > +			clocksource_select();
> >> > 
> >> > Unlikely, but if we have 3 watched clocksources which have the HRES
> >> > bit set, you'd call 3 times clocksource_select().
> > Also the reselect must be called outside the watchdog_lock region.
> 
> Sorry for stupid, the watchdog_lock used protect watchdog related
> resource. but clocksource_select doesn't touch them. So, I know it isn't
> necessary to put reselect under this lock. Just don't know why the
> reselect *must* be called outside of it?

  clocksource_select()
    timekeeping_notify()
      stop_machine()
        get_online_cpus()
	  might_sleep()
	  mutex_lock()

So we need to be in preemptible context when we call clocksource_select().

Thanks,

	tglx
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