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=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_2 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 552F2C11F65 for ; Wed, 30 Jun 2021 08:44:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 376A961D05 for ; Wed, 30 Jun 2021 08:44:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233543AbhF3Iq3 convert rfc822-to-8bit (ORCPT ); Wed, 30 Jun 2021 04:46:29 -0400 Received: from relay6-d.mail.gandi.net ([217.70.183.198]:52981 "EHLO relay6-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232984AbhF3Iq2 (ORCPT ); Wed, 30 Jun 2021 04:46:28 -0400 Received: (Authenticated sender: miquel.raynal@bootlin.com) by relay6-d.mail.gandi.net (Postfix) with ESMTPSA id BF226C0009; Wed, 30 Jun 2021 08:43:54 +0000 (UTC) Date: Wed, 30 Jun 2021 10:43:53 +0200 From: Miquel Raynal To: Desmond Cheong Zhi Xi Cc: richard@nod.at, vigneshr@ti.com, linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org, skhan@linuxfoundation.org, gregkh@linuxfoundation.org, linux-kernel-mentees@lists.linuxfoundation.org, syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com, Christoph Hellwig Subject: Re: [PATCH v2] mtd: break circular locks in register_mtd_blktrans Message-ID: <20210630104353.7575e920@xps13> In-Reply-To: <03e19ec8-7479-9be2-3563-a2fcf9d0ec0c@gmail.com> References: <20210617160904.570111-1-desmondcheongzx@gmail.com> <03e19ec8-7479-9be2-3563-a2fcf9d0ec0c@gmail.com> Organization: Bootlin X-Mailer: Claws Mail 3.17.7 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Desmond Cheong Zhi Xi wrote on Wed, 30 Jun 2021 16:04:17 +0800: > On 18/6/21 12:09 am, Desmond Cheong Zhi Xi wrote: > > Syzbot reported a circular locking dependency: > > https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb > > > > This happens because of the following lock dependencies: > > > > 1. loop_ctl_mutex -> bdev->bd_mutex (when loop_control_ioctl calls > > loop_remove, which then calls del_gendisk; this also happens in > > loop_exit which eventually calls loop_remove) > > > > 2. bdev->bd_mutex -> mtd_table_mutex (when blkdev_get_by_dev calls > > __blkdev_get, which then calls blktrans_open) > > > > 3. mtd_table_mutex -> major_names_lock (when register_mtd_blktrans > > calls __register_blkdev) > > > > 4. major_names_lock -> loop_ctl_mutex (when blk_request_module calls > > loop_probe) > > > > Hence there's an overall dependency of: > > > > loop_ctl_mutex ----------> bdev->bd_mutex > > ^ | > > | | > > | v > > major_names_lock <--------- mtd_table_mutex > > > > We can break this circular dependency by holding mtd_table_mutex only > > for the required critical section in register_mtd_blktrans. This > > avoids the mtd_table_mutex -> major_names_lock dependency. > > > > Reported-and-tested-by: syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com > > Co-developed-by: Christoph Hellwig > > Signed-off-by: Christoph Hellwig > > Signed-off-by: Desmond Cheong Zhi Xi > > --- > > > > Changes in v1 -> v2: > > > > Break the circular dependency in register_mtd_blktrans instead of blk_request_module, as suggested by Christoph Hellwig. > > > > drivers/mtd/mtd_blkdevs.c | 8 ++------ > > 1 file changed, 2 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c > > index fb8e12d590a1..7d26cfe24d05 100644 > > --- a/drivers/mtd/mtd_blkdevs.c > > +++ b/drivers/mtd/mtd_blkdevs.c > > @@ -528,14 +528,10 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr) > > if (!blktrans_notifier.list.next) > > register_mtd_user(&blktrans_notifier); > > > - > > - mutex_lock(&mtd_table_mutex); > > - > > ret = register_blkdev(tr->major, tr->name); > > if (ret < 0) { > > printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n", > > tr->name, tr->major, ret); > > - mutex_unlock(&mtd_table_mutex); > > return ret; > > } > > > @@ -545,12 +541,12 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr) > > tr->blkshift = ffs(tr->blksize) - 1; > > > INIT_LIST_HEAD(&tr->devs); > > - list_add(&tr->list, &blktrans_majors); > > > + mutex_lock(&mtd_table_mutex); > > + list_add(&tr->list, &blktrans_majors); > > mtd_for_each_device(mtd) > > if (mtd->type != MTD_ABSENT) > > tr->add_mtd(tr, mtd); > > - > > mutex_unlock(&mtd_table_mutex); > > return 0; > > } > > > > Hi maintainers, > > Any chance to review this patch? > > For additional reference, the mtd_table_mutex --> major_names_lock hierarchy that can be removed by this patch also appears in a different lock chain: > https://syzkaller.appspot.com/bug?id=cbf5fe846f14a90f05e10df200b08c57941dc750 I'm fine with the patch, but it came too late in the release cycle so now I'm waiting -rc1 to apply it. Thanks, Miquèl