* [PATCH] virtio: remove overzealous BUG_ON.
@ 2008-04-07 4:30 Rusty Russell
0 siblings, 0 replies; 3+ messages in thread
From: Rusty Russell @ 2008-04-07 4:30 UTC (permalink / raw)
To: Balaji Rao, Linus Torvalds; +Cc: virtualization, linux-kernel
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>
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.
*/
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] virtio: remove overzealous BUG_ON.
2008-04-07 13:50 ` Anthony Liguori
@ 2008-04-07 20:09 ` Rusty Russell
0 siblings, 0 replies; 3+ messages in thread
From: Rusty Russell @ 2008-04-07 20:09 UTC (permalink / raw)
To: virtualization
Cc: Anthony Liguori, virtualization, Linus Torvalds, linux-kernel
On Monday 07 April 2008 23:50:24 Anthony Liguori wrote:
> Rusty Russell wrote:
> > 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.
>
> I think the printk() in start_xmit should be removed too (or at least
> rate limited).
Yep, please send patch. I'm about to jump on a plane, so consider this a
pre-canned ack.
Thanks,
Rusty.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] virtio: remove overzealous BUG_ON.
[not found] <200804071430.28399.rusty__35361.6672721275$1207542780$gmane$org@rustcorp.com.au>
@ 2008-04-07 13:50 ` Anthony Liguori
2008-04-07 20:09 ` Rusty Russell
0 siblings, 1 reply; 3+ messages in thread
From: Anthony Liguori @ 2008-04-07 13:50 UTC (permalink / raw)
To: Rusty Russell; +Cc: Balaji Rao, Linus Torvalds, linux-kernel, virtualization
Rusty Russell wrote:
> 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.
I think the printk() in start_xmit should be removed too (or at least
rate limited).
Regards,
Anthony Liguori
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2008-04-07 20:12 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2008-04-07 4:30 [PATCH] virtio: remove overzealous BUG_ON Rusty Russell
[not found] <200804071430.28399.rusty__35361.6672721275$1207542780$gmane$org@rustcorp.com.au>
2008-04-07 13:50 ` Anthony Liguori
2008-04-07 20:09 ` Rusty Russell
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