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    SubjectRe: [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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