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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 88759ECDFB8 for ; Wed, 18 Jul 2018 00:58:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3C7602064D for ; Wed, 18 Jul 2018 00:58:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3C7602064D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=huawei.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 S1731531AbeGRBeH (ORCPT ); Tue, 17 Jul 2018 21:34:07 -0400 Received: from szxga04-in.huawei.com ([45.249.212.190]:9638 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730614AbeGRBeG (ORCPT ); Tue, 17 Jul 2018 21:34:06 -0400 Received: from DGGEMS406-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id E19E7B43B4F99; Wed, 18 Jul 2018 08:58:39 +0800 (CST) Received: from [127.0.0.1] (10.177.16.168) by DGGEMS406-HUB.china.huawei.com (10.3.19.206) with Microsoft SMTP Server id 14.3.382.0; Wed, 18 Jul 2018 08:58:39 +0800 Subject: Re: [V9fs-developer] [PATCH v2] net/9p: Fix a deadlock case in the virtio transport To: Dominique Martinet References: <5B4DCD0A.8040600@huawei.com> <20180717114215.GA14414@nautica> <5B4DE09F.5000800@huawei.com> <20180717130720.GA23759@nautica> CC: Andrew Morton , Eric Van Hensbergen , Ron Minnich , Latchesar Ionkov , Linux Kernel Mailing List , From: jiangyiwen Message-ID: <5B4E90BE.5030307@huawei.com> Date: Wed, 18 Jul 2018 08:58:38 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <20180717130720.GA23759@nautica> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.16.168] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018/7/17 21:07, Dominique Martinet wrote: > jiangyiwen wrote on Tue, Jul 17, 2018: >> On 2018/7/17 19:42, Dominique Martinet wrote: >>> >>>> Subject: net/9p: Fix a deadlock case in the virtio transport >>> >>> I hadn't noticed in the v1, but how is that a deadlock fix? >>> The previous code doesn't look like it deadlocks to me, the commit >>> message is more correct. >>> >> >> If cpu is running in the irq context for a long time, >> NMI watchdog will detect the hard lockup in the cpu, >> and then it will cause kernel panic. So I use this >> subject to underline the scenario. > > That's still not a deadlock - fix lockup would be more appropriate? > > Okay. >>> Do we have a guarantee that req_done is only called if there is at least >>> one buf to read? >>> For example, that there isn't two threads queueing the same callback but >>> the first one reads everything and the second has nothing to read? >>> >>> If virtblk_done takes care of setting up a "req_done" bool to only >>> notify waiters if something has been done I'd rather have a reason to do >>> differently, even if you can argue that nothing bad will happen in case >>> of a gratuitous wake_up >>> >> >> Sorry, I don't fully understand what your mean. >> I think even if the ring buffer don't have the data, wakeup operation >> will not cause any other problem, and the loss of performance can be >> ignored. > > I just mean "others do check, why not us?". It's almost free to check if > we had something to read, but if there are many pending read/writes > waiting for a buffer they will all wake up and spin uselessly. > > I've checked other callers of virtqueue_get_buf() and out of 9 that loop > around in a callback then wake another thread up, 6 do check before > waking up, two check that something happened just to print a debug > statement if not (virtio_test and virtgpu) and one doesn't check > (virtio_input); so I guess we wouldn't be the first ones, just not > following the trend. > > But yes, nothing bad will happen, so let's agree to disagree and I'll > defer to others opinion on this > > > Thanks, > Thanks for your reply, you're right, other callers also check whether Virtio ring has data then do wakeup operation, we also should follow the trend. Okay, I will resend the patch later. Thanks, Yiwen.