KVM: irqfd: Make resampler_list an RCU list

It is useful to be able to do read-only traversal of the list of all the
registered irqfd resamplers without locking the resampler_lock mutex.
In particular, we are going to traverse it to search for a resampler
registered for the given irq of an irqchip, and that will be done with
an irqchip spinlock (ioapic->lock) held, so it is undesirable to lock a
mutex in this context. So turn this list into an RCU list.

For protecting the read side, reuse kvm->irq_srcu which is already used
for protecting a number of irq related things (kvm->irq_routing,
irqfd->resampler->list, kvm->irq_ack_notifier_list,
kvm->arch.mask_notifier_list).

Signed-off-by: Dmytro Maluka <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

include/linux/kvm_host.h

···
 	struct {
 		spinlock_t        lock;
 		struct list_head  items;
+		/* resampler_list update side is protected by resampler_lock. */
 		struct list_head  resampler_list;
 		struct mutex      resampler_lock;
 	} irqfds;

include/linux/kvm_irqfd.h

···
 	/*
 	 * Entry in list of kvm->irqfd.resampler_list.  Use for sharing
 	 * resamplers among irqfds on the same gsi.
-	 * Accessed and modified under kvm->irqfds.resampler_lock
+	 * RCU list modified under kvm->irqfds.resampler_lock
 	 */
 	struct list_head link;
 };

virt/kvm/eventfd.c

···
 	synchronize_srcu(&kvm->irq_srcu);
 
 	if (list_empty(&resampler->list)) {
-		list_del(&resampler->link);
+		list_del_rcu(&resampler->link);
 		kvm_unregister_irq_ack_notifier(kvm, &resampler->notifier);
+		/*
+		 * synchronize_srcu(&kvm->irq_srcu) already called
+		 * in kvm_unregister_irq_ack_notifier().
+		 */
 		kvm_set_irq(kvm, KVM_IRQFD_RESAMPLE_IRQ_SOURCE_ID,
 			    resampler->notifier.gsi, 0, false);
 		kfree(resampler);
···
 			resampler->notifier.irq_acked = irqfd_resampler_ack;
 			INIT_LIST_HEAD(&resampler->link);
 
-			list_add(&resampler->link, &kvm->irqfds.resampler_list);
+			list_add_rcu(&resampler->link, &kvm->irqfds.resampler_list);
 			kvm_register_irq_ack_notifier(kvm,
 						      &resampler->notifier);
 			irqfd->resampler = resampler;