linux-kernel - Re: [RFC PATCH 16/22 -v2] add get_monotonic

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Date:	Tue, 15 Jan 2008 20:38:43 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles


On Tue, 15 Jan 2008, Mathieu Desnoyers wrote:
>
> Ok, but what actually insures that the clock->cycle_* reads won't be
> reordered across the clocksource_read() ?

<looks at code>

Hmm, interesting.I didn't notice that clocksource_read() is a static
inline.  I was thinking that since it was passing a pointer to a function,
gcc could not assume that it could move that code across it. But now
looking to see that clocksource_read is simply a static inline that does:

  cs->read();

But still, can gcc assume that it can push loads of unknown origin
variables across function calls? So something like:

static int *glob;

void foo(void) {
	int x;

	x = *glob;

	bar();

	if (x != *glob)
		/* ... */
}

I can't see how any compiler could honestly move the loading of the first
x after the calling of bar(). With glob pointing to some unknown
variable, that may be perfectly fine for bar to modify.


> > >
> > > > +		cycle_raw = clock->cycle_raw;
> > > > +		cycle_last = clock->cycle_last;
> > > > +
> > > > +		/* read clocksource: */
> > > > +		cycle_now = clocksource_read(clock);

So the question here is,can cycle_raw and cycle_last be loaded from
the unknown source that clock points to after the call to
clocksource_read()?

 I'm thinking not.

> > > > +
> > > > +		/* calculate the delta since the last update_wall_time: */
> > > > +		cycle_delta = (cycle_now - cycle_last) & clock->mask;
> > > > +
> > > > +	} while (cycle_raw != clock->cycle_raw ||
> > > > +		 cycle_last != clock->cycle_last);
> > > > +
> > > > +	return cycle_raw + cycle_delta;
> > > > +}


-- Steve

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