Messages in this thread | | | Date | Tue, 15 Jan 2008 20:38:43 -0500 (EST) | From | Steven Rostedt <> | 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
| |