* Marcelo Tosatti <mtosatti@xxxxxxxxxx> [2012-01-17 09:02:11]:
> > +/* Kick vcpu waiting on @lock->head to reach value @ticket */
> > +static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
> > +{
> > + int cpu;
> > + int apicid;
> > +
> > + add_stats(RELEASED_SLOW, 1);
> > +
> > + for_each_cpu(cpu, &waiting_cpus) {
> > + const struct kvm_lock_waiting *w = &per_cpu(lock_waiting, cpu);
> > + if (ACCESS_ONCE(w->lock) == lock &&
> > + ACCESS_ONCE(w->want) == ticket) {
> > + add_stats(RELEASED_SLOW_KICKED, 1);
> > + apicid = per_cpu(x86_cpu_to_apicid, cpu);
> > + kvm_kick_cpu(apicid);
> > + break;
> > + }
> > + }
>
> What prevents a kick from being lost here, if say, the waiter is at
> local_irq_save in kvm_lock_spinning, before the lock/want assignments?
The waiter does check for lock becoming available before actually
sleeping:
+ /*
+ * check again make sure it didn't become free while
+ * we weren't looking.
+ */
+ if (ACCESS_ONCE(lock->tickets.head) == want) {
+ add_stats(TAKEN_SLOW_PICKUP, 1);
+ goto out;
+ }
- vatsa
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