On 09/18, Peter Zijlstra wrote: > > On Fri, Sep 18, 2020 at 12:01:12PM +0200, pet...@infradead.org wrote: > > + u64 sum = per_cpu_sum(*(u64 *)sem->read_count); > > Moo, that doesn't work, we have to do two separate sums.
Or we can re-introduce "atomic_t slow_read_ctr". percpu_up_read_irqsafe(sem) { preempt_disable(); atomic_dec_release(&sem->slow_read_ctr); if (!rcu_sync_is_idle(&sem->rss)) rcuwait_wake_up(&sem->writer); preempt_enable(); } readers_active_check(sem) { unsigned int sum = per_cpu_sum(*sem->read_count) + (unsigned int)atomic_read(&sem->slow_read_ctr); if (sum) return false; ... } Of course, this assumes that atomic_t->counter underflows "correctly", just like "unsigned int". But again, do we really want this? Oleg.