Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead of an explicit ioctl

Michael S. Tsirkin
2009-06-14T11:50:09Z

On Thu, Jun 04, 2009 at 08:48:12AM -0400, Gregory Haskins wrote:
> +static void
> +irqfd_disconnect(struct _irqfd *irqfd)
> +{
> + struct kvm *kvm;
> +
> + mutex_lock(&irqfd->lock);
> +
> + kvm = rcu_dereference(irqfd->kvm);
> + rcu_assign_pointer(irqfd->kvm, NULL);
> +
> + mutex_unlock(&irqfd->lock);
> +
> + if (!kvm)
> + return;
>
> mutex_lock(&kvm->lock);
> - kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
> - kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
> + list_del(&irqfd->list);
> mutex_unlock(&kvm->lock);
> +
> + /*
> + * It is important to not drop the kvm reference until the next grace
> + * period because there might be lockless references in flight up
> + * until then
> + */
> + synchronize_srcu(&irqfd->srcu);
> + kvm_put_kvm(kvm);
> }

So irqfd object will persist after kvm goes away, until eventfd is closed?

>
> static int
> irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> {
> struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
> + unsigned long flags = (unsigned long)key;
>
> - /*
> - * The wake_up is called with interrupts disabled. Therefore we need
> - * to defer the IRQ injection until later since we need to acquire the
> - * kvm->lock to do so.
> - */
> - schedule_work(&irqfd->work);
> + if (flags & POLLIN)
> + /*
> + * The POLLIN wake_up is called with interrupts disabled.
> + * Therefore we need to defer the IRQ injection until later
> + * since we need to acquire the kvm->lock to do so.
> + */
> + schedule_work(&irqfd->inject);
> +
> + if (flags & POLLHUP) {
> + /*
> + * The POLLHUP is called unlocked, so it theoretically should
> + * be safe to remove ourselves from the wqh using the locked
> + * variant of remove_wait_queue()
> + */
> + remove_wait_queue(irqfd->wqh, &irqfd->wait);
> + flush_work(&irqfd->inject);
> + irqfd_disconnect(irqfd);
> +
> + cleanup_srcu_struct(&irqfd->srcu);
> + kfree(irqfd);
> + }
>
> return 0;
> }

And it is removed by this function when eventfd is closed.
But what prevents the kvm module from going away, meanwhile?

--
MST