The 'disable_cb' callback is designed as an optimization to tell the host
we don't need callbacks now. As it is not reliable, the debug check is
overzealous: it can happen on two CPUs at the same time. Document this.

Even if it were reliable, the virtio_net driver doesn't disable
callbacks on transmit so the START_USE/END_USE debugging reentrance
protection can be easily tripped even on UP.

Thanks to Balaji Rao for the bug report and testing.

Signed-off-by: Rusty Russell
CC: Balaji Rao

diff -r 0b3e64be7696 drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c Mon Apr 07 07:40:08 2008 +1000
+++ b/drivers/virtio/virtio_ring.c Mon Apr 07 13:37:03 2008 +1000
@@ -214,10 +215,7 @@ static void vring_disable_cb(struct virt
{
struct vring_virtqueue *vq = to_vvq(_vq);

- START_USE(vq);
- BUG_ON(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT);
vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
- END_USE(vq);
}

static bool vring_enable_cb(struct virtqueue *_vq)
diff -r 0b3e64be7696 include/linux/virtio.h
--- a/include/linux/virtio.h Mon Apr 07 07:40:08 2008 +1000
+++ b/include/linux/virtio.h Mon Apr 07 13:37:03 2008 +1000
@@ -41,6 +41,8 @@ struct virtqueue
* Returns NULL or the "data" token handed to add_buf.
* @disable_cb: disable callbacks
* vq: the struct virtqueue we're talking about.
+ * Note that this is not necessarily synchronous, hence unreliable and only
+ * useful as an optimization.
* @enable_cb: restart callbacks after disable_cb.
* vq: the struct virtqueue we're talking about.
* This re-enables callbacks; it returns "false" if there are pending
@@ -48,7 +50,8 @@ struct virtqueue
* checking for more work, and enabling callbacks.
*
* Locking rules are straightforward: the driver is responsible for
- * locking. No two operations may be invoked simultaneously.
+ * locking. No two operations may be invoked simultaneously, with the exception
+ * of @disable_cb.
*
* All operations can be called in any context.
*/
--
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