linux-kernel - [PATCH AUTOSEL 4.9 1/3] virtio: Improve vq->broken access to avoid any compiler optimization

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Message-Id: <20210824005539.631820-1-sashal@kernel.org>
Date:   Mon, 23 Aug 2021 20:55:37 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Parav Pandit <parav@...dia.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Sasha Levin <sashal@...nel.org>,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH AUTOSEL 4.9 1/3] virtio: Improve vq->broken access to avoid any compiler optimization

From: Parav Pandit <parav@...dia.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@...dia.com>
Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.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 5cad9f41c238..cf7eccfe3469 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1150,7 +1150,7 @@ 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);
 
@@ -1164,7 +1164,9 @@ 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