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From: Parav Pandit via Virtualization <virtualization@lists.linux-foundation.org>
To: <mst@redhat.com>, <virtualization@lists.linux-foundation.org>
Subject: [PATCH v1 1/4] virtio: Improve vq->broken access to avoid any compiler optimization
Date: Mon, 19 Jul 2021 10:52:15 +0300	[thread overview]
Message-ID: <20210719075218.1485812-2-parav@nvidia.com> (raw)
In-Reply-To: <20210719075218.1485812-1-parav@nvidia.com>

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.

Signed-off-by: Parav Pandit <parav@nvidia.com>
---
changelog:
v0->v1:
 - removed fixes tag from commit log
 - using WRITE_ONCE() as its enough for smp cache coherency as well as
   undesired compiler optimization
---
 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..e179c7c7622c 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);
-- 
2.27.0

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  reply	other threads:[~2021-07-19  7:52 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-19  7:52 [PATCH v1 0/4] virtio short improvements Parav Pandit via Virtualization
2021-07-19  7:52 ` Parav Pandit via Virtualization [this message]
2021-07-19  7:52 ` [PATCH v1 2/4] virtio: Keep vring_del_virtqueue() mirror of VQ create Parav Pandit via Virtualization
2021-07-19  7:52 ` [PATCH v1 3/4] virtio: Protect vqs list access Parav Pandit via Virtualization
2021-07-19  7:52 ` [PATCH v1 4/4] virtio_pci: Support surprise removal of virtio pci device Parav Pandit via Virtualization

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