Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Steven Rostedt
2008-01-16T01:39:20Z

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