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Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul Mundt <lethal@linux-sh.org>, Daniel Walker <dwalker@fifo99.com>,
       Linus Walleij <linus.ml.walleij@gmail.com>, Ingo Molnar <mingo@elte.hu>,
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       John Stultz <johnstul@linux.vnet.ibm.com>
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Date: Thu, 28 May 2009 14:40:16 +0200
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On Thu, 2009-05-28 at 14:22 +0200, Thomas Gleixner wrote:
> On Thu, 28 May 2009, Paul Mundt wrote:
> > @@ -437,10 +441,19 @@ void clocksource_unregister(struct clocksource *cs)
> >  	unsigned long flags;
> >  
> >  	spin_lock_irqsave(&clocksource_lock, flags);
> > +
> > +	if (sched_clocksource == cs) {
> > +		printk(KERN_WARNING "Refusing to unregister "
> > +		       "scheduler clocksource %s", cs->name);
> 
>   Hmm, isn't that dangerous ? The clocksource might be in a module or
>   in kmalloced memory which is going to be freed.

One could play dirty tricks with 'leaking' module ref counts and such so
as to avoid that. The alternative is making sched_clock() look something
like:

u64 __weak sched_clock(void)
{
  struct clocksource *clock;
  u64 time;

  rcu_read_lock();
  clock = rcu_dereference(sched_clocksource);
  time = cyc2ns(clock, read_clocksource(clock));
  rcu_read_unlock;

  return time;
}

and make the module unload do a sync_rcu -- but this might be a little
overkill