From: Parav Pandit <parav@xxxxxxxxxx> [ 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@xxxxxxxxxx> Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav@xxxxxxxxxx Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- drivers/virtio/virtio_ring.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index 51278f8bd3ab..22a4329ed200 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -1198,7 +1198,7 @@ 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); @@ -1212,7 +1212,9 @@ void virtio_break_device(struct virtio_device *dev) 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); } } EXPORT_SYMBOL_GPL(virtio_break_device); -- 2.30.2
