From: Parav Pandit Date: Wed, 21 Jul 2021 14:26:45 +0000 (+0300) Subject: virtio: Improve vq->broken access to avoid any compiler optimization X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=60f0779862e4ab943810187752c462e85f5fa371;p=linux.git virtio: Improve vq->broken access to avoid any compiler optimization 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 Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav@nvidia.com Signed-off-by: Michael S. Tsirkin --- diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index 89bfe46a8a7fa..e179c7c7622cd 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -2373,7 +2373,7 @@ 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); @@ -2387,7 +2387,9 @@ 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);