From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after_eq(). Date: Thu, 17 Oct 2013 11:31:27 +0100 Message-ID: <525FBC7F.9040800@citrix.com> References: <1381944167-24918-1-git-send-email-jianhai.luan@oracle.com> <525FBB4F02000078000FBB30@nat28.tlf.novell.com> <525FA79F.8060601@oracle.com> <525FAABE.5080806@citrix.com> <525FB9BC.9010608@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: Jan Beulich , , , , , To: jianhai luan Return-path: Received: from smtp02.citrix.com ([66.165.176.63]:2250 "EHLO SMTP02.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751152Ab3JQKba (ORCPT ); Thu, 17 Oct 2013 06:31:30 -0400 In-Reply-To: <525FB9BC.9010608@oracle.com> Sender: netdev-owner@vger.kernel.org List-ID: On 17/10/13 11:19, jianhai luan wrote: > > On 2013-10-17 17:15, David Vrabel wrote: >> On 17/10/13 10:02, jianhai luan wrote: >>> On 2013-10-17 16:26, Jan Beulich wrote: >>>>>>> On 16.10.13 at 19:22, Jason Luan wrote: >>>>> time_after_eq() only works if the delta is < MAX_ULONG/2. >>>>> >>>>> If netfront sends at a very low rate, the time between subsequent >>>>> calls >>>>> to tx_credit_exceeded() may exceed MAX_ULONG/2 and the test for >>>>> timer_after_eq() will be incorrect. Credit will not be replenished >>>>> and >>>>> the guest may become unable to send (e.g., if prior to the long >>>>> gap, all >>>>> credit was exhausted). >>>>> >>>>> We should add the scenario which now beyond next_credit+MAX_UNLONG/2. >>>>> Because >>>>> the fact now must be not before than expire, time_before(now, expire) >>>>> == true >>>>> will verify the scenario. >>>>> time_after_eq(now, next_credit) || time_before (now, expire) >>>>> == >>>>> !time_in_range_open(now, expire, next_credit) >>>> So first of all this must be with a 32-bit netback. And the not >>>> coverable gap between activity is well over 240 days long. _If_ >>>> this really needs dealing with, then why is extending this from >>>> 240+ to 480+ days sufficient? I.e. why don't you simply >>>> change to 64-bit jiffy values, and use time_after_eq64()? >>> Yes, the issue only can be reproduced in 32-bit Dom0 (Beyond >>> MAX_ULONG/2 in 64-bit will need long long time) >>> >>> I think the gap should be think all environment even now extending 480+. >>> if now fall in the gap, one timer will be pending and replenish will be >>> in time. Please run the attachment test program. >>> >>> If use time_after_eq64(), expire ,next_credit and other member will must >>> be u64. >> Yes, you'll need to store next_credit as a u64 in vif instead of >> calculating it in tx_credit_exceeded from expires (which is only an >> unsigned long). > > I know that. Even we use u64, time_after_eq() will also do wrong judge > in theory (not in reality because need long long time). If jiffies_64 has millisecond resolution that would be more than 500,000,000 years. > I think the two better fixed way is below: > - By time_before() to judge if now beyond MAX_ULONG/2 This is broken, so no. David