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

* Marcelo Tosatti [off-list ref] [2012-01-17 09:02:11]:

quoted

+/* 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