[PATCH v3 1/2] KVM: irqfd: Make resampler_list an RCU list

[Dmytro Maluka <dmy@semihalf.com>]

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 <dmy@semihalf.com>
---
 include/linux/kvm_host.h  | 1 +
 include/linux/kvm_irqfd.h | 2 +-
 virt/kvm/eventfd.c        | 8 ++++++--
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1c480b1821e1..ee6d906e0138 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -738,6 +738,7 @@ struct kvm {
 	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;
diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index dac047abdba7..8ad43692e3bb 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -31,7 +31,7 @@ struct kvm_kernel_irqfd_resampler {
 	/*
 	 * 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;
 };
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 2a3ed401ce46..61aea70dd888 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -96,8 +96,12 @@ irqfd_resampler_shutdown(struct kvm_kernel_irqfd *irqfd)
 	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);
@@ -369,7 +373,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 			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;
-- 
2.37.1.595.g718a3a8f04-goog