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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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