Commit be9b79e8 authored Jul 21, 2021 by Parav Pandit Committed by Greg Kroah-Hartman Sep 03, 2021

virtio: Improve vq->broken access to avoid any compiler optimization



[ Upstream commit 60f0779862e4ab943810187752c462e85f5fa371 ]

Currently vq->broken field is read by virtqueue_is_broken() in busy
loop in one context by virtnet_send_command().

vq->broken is set to true in other process context by
virtio_break_device(). Reader and writer are accessing it without any
synchronization. This may lead to a compiler optimization which may
result to optimize reading vq->broken only once.

Hence, force reading vq->broken on each invocation of
virtqueue_is_broken() and also force writing it so that such
update is visible to the readers.

It is a theoretical fix that isn't yet encountered in the field.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav@nvidia.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 0d4ba693

drivers/virtio/virtio_ring.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -2268,7 +2268,7 @@ bool virtqueue_is_broken(struct virtqueue *_vq)
		{
		struct vring_virtqueue *vq = to_vvq(_vq);

		return vq->broken;
		return READ_ONCE(vq->broken);
		}
		EXPORT_SYMBOL_GPL(virtqueue_is_broken);

		@@ -2283,7 +2283,9 @@ void virtio_break_device(struct virtio_device *dev)
		spin_lock(&dev->vqs_list_lock);
		list_for_each_entry(_vq, &dev->vqs, list) {
		struct vring_virtqueue *vq = to_vvq(_vq);
		vq->broken = true;

		/* Pairs with READ_ONCE() in virtqueue_is_broken(). */
		WRITE_ONCE(vq->broken, true);
		}
		spin_unlock(&dev->vqs_list_lock);
		}