From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 88BD1C65BAF for ; Wed, 12 Dec 2018 03:40:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 447542086D for ; Wed, 12 Dec 2018 03:40:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 447542086D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726382AbeLLDks (ORCPT ); Tue, 11 Dec 2018 22:40:48 -0500 Received: from mx1.redhat.com ([209.132.183.28]:42940 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726220AbeLLDkr (ORCPT ); Tue, 11 Dec 2018 22:40:47 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id E1B72A4026; Wed, 12 Dec 2018 03:40:46 +0000 (UTC) Received: from redhat.com (ovpn-120-67.rdu2.redhat.com [10.10.120.67]) by smtp.corp.redhat.com (Postfix) with ESMTP id 16908600C9; Wed, 12 Dec 2018 03:40:45 +0000 (UTC) Date: Tue, 11 Dec 2018 22:40:45 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Tonghao Zhang Subject: Re: [PATCH net 2/4] vhost_net: rework on the lock ordering for busy polling Message-ID: <20181211224024-mutt-send-email-mst@kernel.org> References: <20181210094454.21144-1-jasowang@redhat.com> <20181210094454.21144-3-jasowang@redhat.com> <20181210203119-mutt-send-email-mst@kernel.org> <20181210230106-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Wed, 12 Dec 2018 03:40:47 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 12, 2018 at 11:03:57AM +0800, Jason Wang wrote: > > On 2018/12/11 下午12:04, Michael S. Tsirkin wrote: > > On Tue, Dec 11, 2018 at 11:06:43AM +0800, Jason Wang wrote: > > > On 2018/12/11 上午9:34, Michael S. Tsirkin wrote: > > > > On Mon, Dec 10, 2018 at 05:44:52PM +0800, Jason Wang wrote: > > > > > When we try to do rx busy polling in tx path in commit 441abde4cd84 > > > > > ("net: vhost: add rx busy polling in tx path"), we lock rx vq mutex > > > > > after tx vq mutex is held. This may lead deadlock so we try to lock vq > > > > > one by one in commit 78139c94dc8c ("net: vhost: lock the vqs one by > > > > > one"). With this commit, we avoid the deadlock with the assumption > > > > > that handle_rx() and handle_tx() run in a same process. But this > > > > > commit remove the protection for IOTLB updating which requires the > > > > > mutex of each vq to be held. > > > > > > > > > > To solve this issue, the first step is to have a exact same lock > > > > > ordering for vhost_net. This is done through: > > > > > > > > > > - For handle_rx(), if busy polling is enabled, lock tx vq immediately. > > > > > - For handle_tx(), always lock rx vq before tx vq, and unlock it if > > > > > busy polling is not enabled. > > > > > - Remove the tricky locking codes in busy polling. > > > > > > > > > > With this, we can have a exact same lock ordering for vhost_net, this > > > > > allows us to safely revert commit 78139c94dc8c ("net: vhost: lock the > > > > > vqs one by one") in next patch. > > > > > > > > > > The patch will add two more atomic operations on the tx path during > > > > > each round of handle_tx(). 1 byte TCP_RR does not notice such > > > > > overhead. > > > > > > > > > > Fixes: commit 78139c94dc8c ("net: vhost: lock the vqs one by one") > > > > > Cc: Tonghao Zhang > > > > > Signed-off-by: Jason Wang > > > > > --- > > > > > drivers/vhost/net.c | 18 +++++++++++++++--- > > > > > 1 file changed, 15 insertions(+), 3 deletions(-) > > > > > > > > > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > > > > > index ab11b2bee273..5f272ab4d5b4 100644 > > > > > --- a/drivers/vhost/net.c > > > > > +++ b/drivers/vhost/net.c > > > > > @@ -513,7 +513,6 @@ static void vhost_net_busy_poll(struct vhost_net *net, > > > > > struct socket *sock; > > > > > struct vhost_virtqueue *vq = poll_rx ? tvq : rvq; > > > > > - mutex_lock_nested(&vq->mutex, poll_rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX); > > > > > vhost_disable_notify(&net->dev, vq); > > > > > sock = rvq->private_data; > > > > > @@ -543,8 +542,6 @@ static void vhost_net_busy_poll(struct vhost_net *net, > > > > > vhost_net_busy_poll_try_queue(net, vq); > > > > > else if (!poll_rx) /* On tx here, sock has no rx data. */ > > > > > vhost_enable_notify(&net->dev, rvq); > > > > > - > > > > > - mutex_unlock(&vq->mutex); > > > > > } > > > > > static int vhost_net_tx_get_vq_desc(struct vhost_net *net, > > > > > @@ -913,10 +910,16 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock) > > > > > static void handle_tx(struct vhost_net *net) > > > > > { > > > > > struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX]; > > > > > + struct vhost_net_virtqueue *nvq_rx = &net->vqs[VHOST_NET_VQ_RX]; > > > > > struct vhost_virtqueue *vq = &nvq->vq; > > > > > + struct vhost_virtqueue *vq_rx = &nvq_rx->vq; > > > > > struct socket *sock; > > > > > + mutex_lock_nested(&vq_rx->mutex, VHOST_NET_VQ_RX); > > > > > mutex_lock_nested(&vq->mutex, VHOST_NET_VQ_TX); > > > > > + if (!vq->busyloop_timeout) > > > > > + mutex_unlock(&vq_rx->mutex); > > > > > + > > > > > sock = vq->private_data; > > > > > if (!sock) > > > > > goto out; > > > > > @@ -933,6 +936,8 @@ static void handle_tx(struct vhost_net *net) > > > > > handle_tx_copy(net, sock); > > > > > out: > > > > > + if (vq->busyloop_timeout) > > > > > + mutex_unlock(&vq_rx->mutex); > > > > > mutex_unlock(&vq->mutex); > > > > > } > > > > So rx mutex taken on tx path now. And tx mutex is on rc path ... This > > > > is just messed up. Why can't tx polling drop rx lock before > > > > getting the tx lock and vice versa? > > > > > > Because we want to poll both tx and rx virtqueue at the same time > > > (vhost_net_busy_poll()). > > > > > >     while (vhost_can_busy_poll(endtime)) { > > >         if (vhost_has_work(&net->dev)) { > > >             *busyloop_intr = true; > > >             break; > > >         } > > > > > >         if ((sock_has_rx_data(sock) && > > >              !vhost_vq_avail_empty(&net->dev, rvq)) || > > >             !vhost_vq_avail_empty(&net->dev, tvq)) > > >             break; > > > > > >         cpu_relax(); > > > > > >     } > > > > > > > > > And we disable kicks and notification for better performance. > > Right but it's all slow path - it happens when queue is > > otherwise empty. So this is what I am saying: let's drop the locks > > we hold around this. > > > Is this really safe? I looks to me it can race with SET_VRING_ADDR. And the > codes did more: > > - access sock object > > - access device IOTLB > > - enable and disable notification > > None of above is safe without the protection of vq mutex. ys but take another lock. just not nested. > > > > > > > > > Or if we really wanted to force everything to be locked at > > > > all times, let's just use a single mutex. > > > > > > > > > > > > > > > We could, but it might requires more changes which could be done for -next I > > > believe. > > > > > > > > > Thanks > > I'd rather we kept the fine grained locking. E.g. people are > > looking at splitting the tx and rx threads. But if not possible > > let's fix it cleanly with a coarse-grained one. A mess here will > > just create more trouble later. > > > > I believe we won't go back to coarse one. Looks like we can solve this by > using mutex_trylock() for rxq during TX. And don't do polling for rxq is a > IOTLB updating is pending. > > Let me post V2. > > Thanks