From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: [PATCH net-next 4/6] net: hip04: Don't free the tx skb when the buffer is not ready for xmit Date: Thu, 16 Apr 2015 10:41:09 +0200 Message-ID: <4971105.FJGVdDRE9f@wuerfel> References: <1429101008-9464-1-git-send-email-dingtianhong@huawei.com> <2218546.M1hzu1bzfh@wuerfel> <552F560D.4070309@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Cc: davem@davemloft.net, grant.likely@linaro.org, sergei.shtylyov@cogentembedded.com, linux-arm-kernel@lists.infradead.org, eric.dumazet@gmail.com, zhangfei.gao@linaro.org, joe@perches.com, netdev@vger.kernel.org, devicetree@vger.kernel.org, linux@arm.linux.org.uk To: Ding Tianhong Return-path: Received: from mout.kundenserver.de ([212.227.17.13]:60769 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750755AbbDPIlU (ORCPT ); Thu, 16 Apr 2015 04:41:20 -0400 In-Reply-To: <552F560D.4070309@huawei.com> Sender: netdev-owner@vger.kernel.org List-ID: On Thursday 16 April 2015 14:26:21 Ding Tianhong wrote: > On 2015/4/15 22:19, Arnd Bergmann wrote: > > On Wednesday 15 April 2015 20:30:06 Ding Tianhong wrote: > >> @@ -489,6 +487,8 @@ static int hip04_mac_start_xmit(struct sk_buff *skb, struct net_device *ndev) > >> > >> /* Ensure tx_head update visible to tx reclaim */ > >> smp_wmb(); > >> + count++; > >> + priv->tx_head = TX_NEXT(tx_head); > >> > >> > > > > Something is wrong here, the comment does not match the code any more, because > > the smp_wmb() won't actually make the tx_head update visible. > > > > Arnd > > > Yes, the smp_wmb() could only make sure the tx buffer was ready to xmit. The problem you need to avoid is that the tx reclaim function thinks it's done with all outstanding packets and does not retrigger, while the start_xmit thinks it will still get to that. This means you need a barrier between the priv->tx_head update and the napi_schedule_prep() call. Arnd From mboxrd@z Thu Jan 1 00:00:00 1970 From: arnd@arndb.de (Arnd Bergmann) Date: Thu, 16 Apr 2015 10:41:09 +0200 Subject: [PATCH net-next 4/6] net: hip04: Don't free the tx skb when the buffer is not ready for xmit In-Reply-To: <552F560D.4070309@huawei.com> References: <1429101008-9464-1-git-send-email-dingtianhong@huawei.com> <2218546.M1hzu1bzfh@wuerfel> <552F560D.4070309@huawei.com> Message-ID: <4971105.FJGVdDRE9f@wuerfel> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thursday 16 April 2015 14:26:21 Ding Tianhong wrote: > On 2015/4/15 22:19, Arnd Bergmann wrote: > > On Wednesday 15 April 2015 20:30:06 Ding Tianhong wrote: > >> @@ -489,6 +487,8 @@ static int hip04_mac_start_xmit(struct sk_buff *skb, struct net_device *ndev) > >> > >> /* Ensure tx_head update visible to tx reclaim */ > >> smp_wmb(); > >> + count++; > >> + priv->tx_head = TX_NEXT(tx_head); > >> > >> > > > > Something is wrong here, the comment does not match the code any more, because > > the smp_wmb() won't actually make the tx_head update visible. > > > > Arnd > > > Yes, the smp_wmb() could only make sure the tx buffer was ready to xmit. The problem you need to avoid is that the tx reclaim function thinks it's done with all outstanding packets and does not retrigger, while the start_xmit thinks it will still get to that. This means you need a barrier between the priv->tx_head update and the napi_schedule_prep() call. Arnd