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    SubjectRe: [PATCH 08/14] hrtimer: Allow hrtimer::function() to free the timer
    On Mon, Jun 08, 2015 at 11:14:17AM +0200, Peter Zijlstra wrote:
    > On Mon, Jun 08, 2015 at 12:33:17AM +0200, Oleg Nesterov wrote:
    > > And I think there is a problem. Consider a timer TIMER which always
    > > rearms itself using some "default" timeout.
    > >
    > > In this case __hrtimer_start_range_ns(&TIMER, ...) must preserve
    > > hrtimer_active(&TIMER) == T. By definition, and currently this is
    > > true.
    > >
    > > After this patch this is no longer true (afaics). If the timer is
    > > pending but not running, __hrtimer_start_range_ns()->remove_hrtimer()
    > > will clear ENQUEUED first, then set it again in enqueue_hrtimer().
    >
    > That is so even with the current code; the current code uses:
    >
    > hrtimer->state & CALLBACK
    >
    > for __remove_hrtimer(.state). In the above case of a pending timer,
    > that's 0 aka. HRTIMER_STATE_INACTIVE.
    >
    > > This means that hrtimer_active() returns false in between. And note
    > > that it doesn't matter if the timer changes its ->base or not, so
    > > that 2nd cpu_base above can't help.
    > >
    > > I think that __hrtimer_start_range_ns() should preserve ENQUEUED
    > > like migrate_hrtimer_list() should do (see the previous email).
    >
    > I tend to agree, but I think its a pre-existing problem, not one
    > introduced by my proposed patch.

    Something like this would fix that I think. It fully preserves
    timer->state over hrtimer_start_range_ns().

    ---
    kernel/time/hrtimer.c | 23 +++++++++++++----------
    1 file changed, 13 insertions(+), 10 deletions(-)

    --- a/kernel/time/hrtimer.c
    +++ b/kernel/time/hrtimer.c
    @@ -891,10 +891,10 @@ static void __remove_hrtimer(struct hrti
    * remove hrtimer, called with base lock held
    */
    static inline int
    -remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base)
    +remove_hrtimer(struct hrtimer *timer, struct hrtimer_clock_base *base, bool restart)
    {
    if (hrtimer_is_queued(timer)) {
    - unsigned long state;
    + unsigned long state = timer->state;
    int reprogram;

    /*
    @@ -908,12 +908,15 @@ remove_hrtimer(struct hrtimer *timer, st
    debug_deactivate(timer);
    timer_stats_hrtimer_clear_start_info(timer);
    reprogram = base->cpu_base == this_cpu_ptr(&hrtimer_bases);
    - /*
    - * We must preserve the CALLBACK state flag here,
    - * otherwise we could move the timer base in
    - * switch_hrtimer_base.
    - */
    - state = timer->state & HRTIMER_STATE_CALLBACK;
    +
    + if (!restart) {
    + /*
    + * We must preserve the CALLBACK state flag here,
    + * otherwise we could move the timer base in
    + * switch_hrtimer_base.
    + */
    + state &= HRTIMER_STATE_CALLBACK;
    + }
    __remove_hrtimer(timer, base, state, reprogram);
    return 1;
    }
    @@ -938,7 +941,7 @@ void hrtimer_start_range_ns(struct hrtim
    base = lock_hrtimer_base(timer, &flags);

    /* Remove an active timer from the queue: */
    - remove_hrtimer(timer, base);
    + remove_hrtimer(timer, base, true);

    if (mode & HRTIMER_MODE_REL) {
    tim = ktime_add_safe(tim, base->get_time());
    @@ -1007,7 +1010,7 @@ int hrtimer_try_to_cancel(struct hrtimer
    base = lock_hrtimer_base(timer, &flags);

    if (!hrtimer_callback_running(timer))
    - ret = remove_hrtimer(timer, base);
    + ret = remove_hrtimer(timer, base, false);

    unlock_hrtimer_base(timer, &flags);


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