Sasha Levin
2021-Aug-24 00:55 UTC
[PATCH AUTOSEL 4.14 2/7] virtio: Improve vq->broken access to avoid any compiler optimization
From: Parav Pandit <parav at nvidia.com>
[ 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 at nvidia.com>
Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav at nvidia.com
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
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