On Tue, Dec 19, 2017 at 09:58:44AM +0100, Sebastian Andrzej Siewior wrote: > this is late I know… > > On 2017-09-27 18:40:26 [+0200], Peter Zijlstra wrote: > > - removed superfluous local_bh_disable(), since local_irq_disable() > > already implies much the same. > > it is not superfluous. > > > Please consider... > > > > @@ -1768,7 +1786,6 @@ int hrtimers_dead_cpu(unsigned int scpu) > > BUG_ON(cpu_online(scpu)); > > tick_cancel_sched_timer(scpu); > > > > - local_bh_disable(); > > local_irq_disable(); > > old_base = &per_cpu(hrtimer_bases, scpu); > > new_base = this_cpu_ptr(&hrtimer_bases); > > @@ -1796,7 +1813,6 @@ int hrtimers_dead_cpu(unsigned int scpu) > > /* Check, if we got expired work to do */ > > __hrtimer_peek_ahead_timers(); > > local_irq_enable(); > > - local_bh_enable(); > > return 0; > > } > > we need in there. That local_bh_disable() is required in order to let > raise_softirq_irqoff() not do anything stupid in particular we need > !in_interrupt() defer wakeup_softirqd() until local_bh_enable(). > Otherwise wakeup_softirqd() might actually try to wakeup the process and > go after the pi_lock which can't happen while holding cpu_base->lock.
Argh, that's horrible and definitely needs a comment.