virtio: remove overzealous BUG_ON.

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 <rusty@rustcorp.com.au>
CC: Balaji Rao <balajirrao@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/virtio/virtio_ring.c

@@ -214,10 +214,7 @@ static void vring_disable_cb(struct virtqueue *_vq)
 {
 	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)

include/linux/virtio.h

@@ -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.
  */