# \[PATCH v3 29/41\] Revert "virtio\_ring: Update weak barriers to use dma\_wmb/rmb"

- From: Michael S. Tsirkin \<mst@redhat.com\>
- Date: 2016-01-10 14:20:46 
- List: linux-arm-kernel
- Page: https://ratatoskr.run/linux-arm-kernel/2016/01/4656824
- Thread: https://ratatoskr.run/linux-arch/2016/01/4656797/t.md

```text
This reverts commit 9e1a27ea42691429e31f158cce6fc61bc79bb2e9.

While that commit optimizes !CONFIG_SMP, it mixes
up DMA and SMP concepts, making the code hard
to figure out.

A better way to optimize this is with the new __smp_XXX
barriers.

As a first step, go back to full rmb/wmb barriers
for !SMP.
We switch to __smp_XXX barriers in the next patch.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alexander Duyck <redacted>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/virtio_ring.h | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 8e50888..67e06fe 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -21,20 +21,19 @@
  * actually quite cheap.
  */
 
+#ifdef CONFIG_SMP
 static inline void virtio_mb(bool weak_barriers)
 {
-#ifdef CONFIG_SMP
 	if (weak_barriers)
 		smp_mb();
 	else
-#endif
 		mb();
 }
 
 static inline void virtio_rmb(bool weak_barriers)
 {
 	if (weak_barriers)
-		dma_rmb();
+		smp_rmb();
 	else
 		rmb();
 }
@@ -42,10 +41,26 @@ static inline void virtio_rmb(bool weak_barriers)
 static inline void virtio_wmb(bool weak_barriers)
 {
 	if (weak_barriers)
-		dma_wmb();
+		smp_wmb();
 	else
 		wmb();
 }
+#else
+static inline void virtio_mb(bool weak_barriers)
+{
+	mb();
+}
+
+static inline void virtio_rmb(bool weak_barriers)
+{
+	rmb();
+}
+
+static inline void virtio_wmb(bool weak_barriers)
+{
+	wmb();
+}
+#endif
 
 struct virtio_device;
 struct virtqueue;
-- 
MST
```