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    SubjectRe: [patch 04/15] clocksource: Add module refcount
    On 04/25/2013 01:31 PM, Thomas Gleixner wrote:
    > Add a module refcount, so the current clocksource cannot be removed
    > unconditionally.
    >
    > Signed-off-by: Thomas Gleixner<tglx@linutronix.de>
    > ---
    > include/linux/clocksource.h | 3 +++
    > kernel/time/timekeeping.c | 15 ++++++++++-----
    > 2 files changed, 13 insertions(+), 5 deletions(-)
    >
    > Index: tip/include/linux/clocksource.h
    > ===================================================================
    > --- tip.orig/include/linux/clocksource.h
    > +++ tip/include/linux/clocksource.h
    > @@ -21,6 +21,7 @@
    > /* clocksource cycle base type */
    > typedef u64 cycle_t;
    > struct clocksource;
    > +struct module;
    >
    > #ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
    > #include <asm/clocksource.h>
    > @@ -162,6 +163,7 @@ extern u64 timecounter_cyc2time(struct t
    > * @suspend: suspend function for the clocksource, if necessary
    > * @resume: resume function for the clocksource, if necessary
    > * @cycle_last: most recent cycle counter value seen by ::read()
    > + * @owner: module reference
    > */
    > struct clocksource {
    > /*
    > @@ -195,6 +197,7 @@ struct clocksource {
    > cycle_t cs_last;
    > cycle_t wd_last;
    > #endif
    > + struct module *owner;

    Maybe could you add a comment about who is expected to set the owner ptr?



    > } ____cacheline_aligned;
    >
    > /*
    > Index: tip/kernel/time/timekeeping.c
    > ===================================================================
    > --- tip.orig/kernel/time/timekeeping.c
    > +++ tip/kernel/time/timekeeping.c
    > @@ -627,11 +627,16 @@ static int change_clocksource(void *data
    > write_seqcount_begin(&timekeeper_seq);
    >
    > timekeeping_forward_now(tk);
    > - if (!new->enable || new->enable(new) == 0) {
    > - old = tk->clock;
    > - tk_setup_internals(tk, new);
    > - if (old->disable)
    > - old->disable(old);
    > + if (try_module_get(new->owner)) {

    Hrm.. So this works, but the interface is a bit non-intuitive, since
    without looking up try_module_get() I'd be surprised that this would
    succeed if the owner ptr is null. Maybe deserves a comment?

    > + if (!new->enable || new->enable(new) == 0) {
    > + old = tk->clock;
    > + tk_setup_internals(tk, new);
    > + if (old->disable)
    > + old->disable(old);
    > + module_put(old->owner);
    > + } else {
    > + module_put(new->owner);
    > + }
    > }
    > timekeeping_update(tk, true, true);
    >

    thanks
    -john



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