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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 7FD5DC3F68F for ; Fri, 3 Jan 2020 07:35:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 607C424649 for ; Fri, 3 Jan 2020 07:35:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725928AbgACHf5 (ORCPT ); Fri, 3 Jan 2020 02:35:57 -0500 Received: from szxga04-in.huawei.com ([45.249.212.190]:8663 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725890AbgACHf5 (ORCPT ); Fri, 3 Jan 2020 02:35:57 -0500 Received: from DGGEMS401-HUB.china.huawei.com (unknown [172.30.72.58]) by Forcepoint Email with ESMTP id F25DE6E83B8FB17F2FD6; Fri, 3 Jan 2020 15:35:52 +0800 (CST) Received: from [127.0.0.1] (10.133.219.224) by DGGEMS401-HUB.china.huawei.com (10.3.19.201) with Microsoft SMTP Server id 14.3.439.0; Fri, 3 Jan 2020 15:35:43 +0800 Subject: Re: [PATCH] block: make sure last_lookup set as NULL after part deleted To: Ming Lei CC: Yufen Yu , , , , , , , , References: <20191231110945.10857-1-yuyufen@huawei.com> <20200102012314.GB16719@ming.t460p> <20200103041805.GA29924@ming.t460p> From: Hou Tao Message-ID: Date: Fri, 3 Jan 2020 15:35:42 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <20200103041805.GA29924@ming.t460p> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.133.219.224] X-CFilter-Loop: Reflected Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi, On 2020/1/3 12:18, Ming Lei wrote: > On Fri, Jan 03, 2020 at 11:06:25AM +0800, Hou Tao wrote: >> Hi, >> >> On 2020/1/2 9:23, Ming Lei wrote: >>> On Tue, Dec 31, 2019 at 10:55:47PM +0800, Hou Tao wrote: >>>> Hi, >>>> snip >> We have got a seemingly better solution: caching the index of last_lookup in tbl->part[] >> instead of caching the pointer itself, so we can ensure the validity of returned pointer >> by ensuring it's not NULL in tbl->part[] as does when last_lookup is NULL or 0. > > Thinking of the problem further, looks we don't need to hold ref for > .last_lookup. > > What we need is to make sure the partition's ref is increased just > before assigning .last_lookup, so how about something like the following? > The approach will work for the above case, but it will not work for the following case: when blk_account_io_done() releases the last ref-counter of last_lookup and calls call_rcu(), and then a RCU read gets the to-be-freed hd-struct. blk_account_io_done rcu_read_lock() // the last ref of last_lookup hd_struct_put() call_rcu rcu_read_lock read last_lookup free() // use-after-free ? hd_struct_try_get Regards, Tao > diff --git a/block/blk-core.c b/block/blk-core.c > index 089e890ab208..79599f5fd5b7 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -1365,18 +1365,6 @@ void blk_account_io_start(struct request *rq, bool new_io) > part_stat_inc(part, merges[rw]); > } else { > part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq)); > - if (!hd_struct_try_get(part)) { > - /* > - * The partition is already being removed, > - * the request will be accounted on the disk only > - * > - * We take a reference on disk->part0 although that > - * partition will never be deleted, so we can treat > - * it as any other partition. > - */ > - part = &rq->rq_disk->part0; > - hd_struct_get(part); > - } > part_inc_in_flight(rq->q, part, rw); > rq->part = part; > } > diff --git a/block/genhd.c b/block/genhd.c > index ff6268970ddc..21f4a9b8d24d 100644 > --- a/block/genhd.c > +++ b/block/genhd.c > @@ -286,17 +286,24 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector) > ptbl = rcu_dereference(disk->part_tbl); > > part = rcu_dereference(ptbl->last_lookup); > - if (part && sector_in_part(part, sector)) > + if (part && sector_in_part(part, sector)) { > + if (!hd_struct_try_get(part)) > + goto exit; > return part; > + } > > for (i = 1; i < ptbl->len; i++) { > part = rcu_dereference(ptbl->part[i]); > > if (part && sector_in_part(part, sector)) { > + if (!hd_struct_try_get(part)) > + goto exit; > rcu_assign_pointer(ptbl->last_lookup, part); > return part; > } > } > + exit: > + hd_struct_get(&disk->part0); > return &disk->part0; > } > EXPORT_SYMBOL_GPL(disk_map_sector_rcu); > > >> >>> Given partition is actually protected by percpu-refcount now, I guess the >>> RCU annotation for referencing ->part[partno] and ->last_lookup may not >>> be necessary, together with the part->rcu_work. >>> >> So we will depends on the invocation of of call_rcu() on __percpu_ref_switch_mode() to >> ensure the RCU readers will find part[i] is NULL before trying to increasing >> the atomic ref-counter of part[i], right ? > > Yeah. > > Thanks, > Ming > > > . >