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Subject: Re: [PATCH] sched: Support current clocksource handling in
 fallback sched_clock().
From: Peter Zijlstra <peterz@infradead.org>
To: Paul Mundt <lethal@linux-sh.org>
Cc: Daniel Walker <dwalker@fifo99.com>, Thomas Gleixner <tglx@linutronix.de>,
       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 11:34:41 +0200
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On Thu, 2009-05-28 at 18:19 +0900, Paul Mundt wrote:

> Ok, there were some ordering problems with the early platform code, but
> I've played with this a bit more and got it to the point where this now
> also works. I can live with this over the v3 version if people prefer
> this approach instead.
> 
> --

> @@ -38,8 +39,7 @@
>   */
>  unsigned long long __attribute__((weak)) sched_clock(void)
>  {
> -	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
> -					* (NSEC_PER_SEC / HZ);
> +	return cyc2ns(sched_clocksource, clocksource_read(sched_clocksource));
>  }
>  
>  static __read_mostly int sched_clock_running;

> @@ -362,6 +363,9 @@ static struct clocksource *select_clocksource(void)
>  	if (next == curr_clocksource)
>  		return NULL;
>  
> +	if (next->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK)
> +		sched_clocksource = next;
> +
>  	return next;
>  }
>  

That's a single assignment, vs two reads on use. Should we be worried
about the SMP race where we observe two different sched_clocksource
pointers on read?


I would suggest we write it as:

u64 __weak sched_clock(void)
{
	struct clocksource *clock = ACCESS_ONCE(sched_clocksource);

	return cyc2ns(clock, clocksource_read(clock));
}