Re: [Xen-devel] [PATCH RFC V4 4/5] kvm : pv-ticketlocks support for linux guests running on KVM hypervisor

[Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>]

* 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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