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    SubjectRe: [PATCH 17/25] hrtimer: Implementation of softirq hrtimer handling
    On Tue, 26 Sep 2017, Peter Zijlstra wrote:

    > On Thu, Aug 31, 2017 at 12:23:42PM -0000, Anna-Maria Gleixner wrote:
    > > static void __run_hrtimer(struct hrtimer_cpu_base *cpu_base,
    > > struct hrtimer_clock_base *base,
    > > - struct hrtimer *timer, ktime_t *now)
    > > + struct hrtimer *timer, ktime_t *now,
    > > + bool hardirq)
    > > {
    > > enum hrtimer_restart (*fn)(struct hrtimer *);
    > > int restart;
    > > @@ -1241,11 +1298,19 @@ static void __run_hrtimer(struct hrtimer
    > > * protected against migration to a different CPU even if the lock
    > > * is dropped.
    > > */
    > > - raw_spin_unlock(&cpu_base->lock);
    > > + if (hardirq)
    > > + raw_spin_unlock(&cpu_base->lock);
    > > + else
    > > + raw_spin_unlock_irq(&cpu_base->lock);
    > > +
    > > trace_hrtimer_expire_entry(timer, now);
    > > restart = fn(timer);
    > > trace_hrtimer_expire_exit(timer);
    > > - raw_spin_lock(&cpu_base->lock);
    > > +
    > > + if (hardirq)
    > > + raw_spin_lock(&cpu_base->lock);
    > > + else
    > > + raw_spin_lock_irq(&cpu_base->lock);
    > >
    >
    > That's just nasty...
    >

    I know and Thomas was unhappy about that as well, but we did not come
    up with a better solution.

    The nasty alternative is:

    static void __run_hrtimer(struct hrtimer_cpu_base *cpu_base,
    struct hrtimer_clock_base *base,
    - struct hrtimer *timer, ktime_t *now)
    + struct hrtimer *timer, ktime_t *now,
    + unsigned long flags)
    ...
    - raw_spin_unlock(&cpu_base->lock);
    + raw_spin_unlock_irqrestore(&cpu_base->lock, flags);

    ...
    - raw_spin_lock(&cpu_base->lock);
    + raw_spin_lock_irq(&cpu_base->lock, flags);

    and hand in flags from the callsites via local_save_flags().

    We wanted to avoid the pointless lock_irq for the interrupt context,
    but yes the conditional is equally bad.


    Anna-Maria

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