Re: [PATCH 1/4] virtio: Improve vq->broken access to avoid any compiler optimization
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2021-07-17 20:38:55
On Sat, Jul 17, 2021 at 10:42:55AM +0300, Parav Pandit wrote:
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 ensure that its update is visible.
Fixes: e2dcdfe95c0b ("virtio: virtio_break_device() to mark all virtqueues broken.")This is all theoretical right? virtqueue_get_buf is not inlined so compiler generally assumes any vq field can change. I'm inclined to not include a Fixes tag then. And please do change subject to say "theoretical" to make that clear to people.
quoted hunk ↗ jump to hunk
Signed-off-by: Parav Pandit <redacted> --- 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 89bfe46a8a7f..7f379fe7d78d 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(). */ + smp_store_release(&vq->broken, true);
A bit puzzled here. Why do we need release semantics here? IUC store_release does not generally pair with READ_ONCE - does it? The commit log does not describe this either.
} } EXPORT_SYMBOL_GPL(virtio_break_device); -- 2.27.0
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