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Wed, 30 Jun 2021 01:04:21 -0700 (PDT) Subject: Re: [PATCH v2] mtd: break circular locks in register_mtd_blktrans To: miquel.raynal@bootlin.com, richard@nod.at, vigneshr@ti.com Cc: 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 References: <20210617160904.570111-1-desmondcheongzx@gmail.com> From: Desmond Cheong Zhi Xi Message-ID: <03e19ec8-7479-9be2-3563-a2fcf9d0ec0c@gmail.com> Date: Wed, 30 Jun 2021 16:04:17 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210617160904.570111-1-desmondcheongzx@gmail.com> Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210630_010424_366905_9784DA62 X-CRM114-Status: GOOD ( 23.59 ) X-BeenThere: linux-mtd@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-mtd" Errors-To: linux-mtd-bounces+linux-mtd=archiver.kernel.org@lists.infradead.org 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 Best wishes, Desmond ______________________________________________________ Linux MTD discussion mailing list http://lists.infradead.org/mailman/listinfo/linux-mtd/