From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753632AbYDGEbC (ORCPT ); Mon, 7 Apr 2008 00:31:02 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750942AbYDGEaw (ORCPT ); Mon, 7 Apr 2008 00:30:52 -0400 Received: from ozlabs.org ([203.10.76.45]:39199 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750831AbYDGEav (ORCPT ); Mon, 7 Apr 2008 00:30:51 -0400 From: Rusty Russell To: Balaji Rao , Linus Torvalds Subject: [PATCH] virtio: remove overzealous BUG_ON. Date: Mon, 7 Apr 2008 14:30:28 +1000 User-Agent: KMail/1.9.9 Cc: virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200804071430.28399.rusty@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. */