From 2557a933b795c1988c721ebb871cd735128bb9cb Mon Sep 17 00:00:00 2001
From: Rusty Russell <rusty@rustcorp.com.au>
Date: Mon, 7 Apr 2008 14:30:28 +1000
Subject: [PATCH] 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 | 3 ---
include/linux/virtio.h | 5 ++++-
2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index aa714028641e..c2fa5c630813 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/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)
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 12c18ac1b973..e7d10845b3c1 100644
--- a/include/linux/virtio.h
+++ b/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.
*/