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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 8D7B4C433DB for ; Fri, 29 Jan 2021 03:37:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6597164DFB for ; Fri, 29 Jan 2021 03:37:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231769AbhA2Dg6 (ORCPT ); Thu, 28 Jan 2021 22:36:58 -0500 Received: from szxga02-in.huawei.com ([45.249.212.188]:2997 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231734AbhA2Dgw (ORCPT ); Thu, 28 Jan 2021 22:36:52 -0500 Received: from DGGEMM403-HUB.china.huawei.com (unknown [172.30.72.55]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4DRjdf47cvzR8Ny; Fri, 29 Jan 2021 11:35:02 +0800 (CST) Received: from dggema772-chm.china.huawei.com (10.1.198.214) by DGGEMM403-HUB.china.huawei.com (10.3.20.211) with Microsoft SMTP Server (TLS) id 14.3.498.0; Fri, 29 Jan 2021 11:36:09 +0800 Received: from [10.169.42.93] (10.169.42.93) by dggema772-chm.china.huawei.com (10.1.198.214) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.2106.2; Fri, 29 Jan 2021 11:36:08 +0800 Subject: Re: [PATCH v2] nvme-multipath: Early exit if no path is available To: Sagi Grimberg , Daniel Wagner CC: , , "Jens Axboe" , Hannes Reinecke , Keith Busch , Christoph Hellwig References: <20210127103033.15318-1-dwagner@suse.de> <20210128075837.u5u56t23fq5gu6ou@beryllium.lan> <69575290-200e-b4a1-4269-c71e4c2cc37b@huawei.com> <20210128094004.erwnszjqcxlsi2kd@beryllium.lan> <675d3cf7-1ae8-adc5-b6d0-359fe10f6b23@grimberg.me> <59cd053e-46cb-0235-141f-4ce919c93f48@huawei.com> From: Chao Leng Message-ID: Date: Fri, 29 Jan 2021 11:36:08 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.169.42.93] X-ClientProxiedBy: dggeme714-chm.china.huawei.com (10.1.199.110) To dggema772-chm.china.huawei.com (10.1.198.214) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/1/29 11:30, Sagi Grimberg wrote: > >>>>> You can't see exactly where it dies but I followed the assembly to >>>>> nvme_round_robin_path(). Maybe it's not the initial nvme_next_ns(head, >>>>> old) which returns NULL but nvme_next_ns() is returning NULL eventually >>>>> (list_next_or_null_rcu()). >>>> So there is other bug cause nvme_next_ns abormal. >>>> I review the code about head->list and head->current_path, I find 2 bugs >>>> may cause the bug: >>>> First, I already send the patch. see: >>>> https://lore.kernel.org/linux-nvme/20210128033351.22116-1-lengchao@huawei.com/ >>>> Second, in nvme_ns_remove, list_del_rcu is before >>>> nvme_mpath_clear_current_path. This may cause "old" is deleted from the >>>> "head", but still use "old". I'm not sure there's any other >>>> consideration here, I will check it and try to fix it. >>> >>> The reason why we first remove from head->list and only then clear >>> current_path is because the other way around there is no way >>> to guarantee that that the ns won't be assigned as current_path >>> again (because it is in head->list). >> ok, I see. >>> >>> nvme_ns_remove fences continue of deletion of the ns by synchronizing >>> the srcu such that for sure the current_path clearance is visible. >> The list will be like this: >> head->next = ns1; >> ns1->next = head; >> old->next = ns1; >> This may cause infinite loop in nvme_round_robin_path. >> for (ns = nvme_next_ns(head, old); >>      ns != old; >>      ns = nvme_next_ns(head, ns)) >> The ns will always be ns1, and then infinite loop. > > Who is being removed? I'm not following The "old" is being removed path. Daniel Wagner report crash like this: head->next = head; old->next = head; So nvme_next_ns(head, old) will return NULL, and then crash. Although check ns can avoid crash, but can not avoid infinite loop. Similar reason, The list will be like this: head->next = ns1; ns1->next = head; old->next = ns1; ns1 is other path. > .