linux-kernel - Re: [PATCH] sched: Support current clocksource handling in fallback sched

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Message-Id: <1243514416.6600.74.camel@laptop>
Date:	Thu, 28 May 2009 14:40:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Paul Mundt <lethal@...ux-sh.org>,
	Daniel Walker <dwalker@...o99.com>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	John Stultz <johnstul@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().

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

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