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

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[ Upstream commit 60f0779862 ]

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>
This commit is contained in:
Parav Pandit
2021-07-21 17:26:45 +03:00
committed by Greg Kroah-Hartman
parent f41c7462d8
commit 065a13c299
1 changed files with 4 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
drivers/virtio/virtio_ring.c
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@@ -2268,7 +2268,7 @@ bool virtqueue_is_broken(struct virtqueue *_vq)
{ {
struct vring_virtqueue *vq = to_vvq(_vq); struct vring_virtqueue *vq = to_vvq(_vq);
return vq->broken; return READ_ONCE(vq->broken);
} }
EXPORT_SYMBOL_GPL(virtqueue_is_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); spin_lock(&dev->vqs_list_lock);
list_for_each_entry(_vq, &dev->vqs, list) { list_for_each_entry(_vq, &dev->vqs, list) {
struct vring_virtqueue *vq = to_vvq(_vq); 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); spin_unlock(&dev->vqs_list_lock);
} }