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=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 6ED94C3A589 for ; Wed, 21 Aug 2019 03:01:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3A7DD22DA7 for ; Wed, 21 Aug 2019 03:01:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726435AbfHUDBI (ORCPT ); Tue, 20 Aug 2019 23:01:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44874 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726193AbfHUDBI (ORCPT ); Tue, 20 Aug 2019 23:01:08 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 90E3C58569; Wed, 21 Aug 2019 03:01:07 +0000 (UTC) Received: from ming.t460p (ovpn-8-25.pek2.redhat.com [10.72.8.25]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 91D5A3D8F; Wed, 21 Aug 2019 03:00:58 +0000 (UTC) Date: Wed, 21 Aug 2019 11:00:53 +0800 From: Ming Lei To: Bart Van Assche Cc: Jens Axboe , linux-block@vger.kernel.org, Greg KH , Mike Snitzer Subject: Re: [PATCH] block: don't acquire .sysfs_lock before removing mq & iosched kobjects Message-ID: <20190821030052.GD24167@ming.t460p> References: <20190816135506.29253-1-ming.lei@redhat.com> <09092247-1623-57ff-6297-1abd9a8cc8a2@acm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <09092247-1623-57ff-6297-1abd9a8cc8a2@acm.org> User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Wed, 21 Aug 2019 03:01:07 +0000 (UTC) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, Aug 20, 2019 at 02:21:10PM -0700, Bart Van Assche wrote: > On 8/16/19 6:55 AM, Ming Lei wrote: > > @@ -567,8 +568,17 @@ int elevator_switch_mq(struct request_queue *q, > > lockdep_assert_held(&q->sysfs_lock); > > if (q->elevator) { > > - if (q->elevator->registered) > > + if (q->elevator->registered) { > > + /* > > + * sysfs write is exclusively, release > > + * sysfs_lock for avoiding deadlock with > > + * sysfs built-in lock which is required > > + * in either .show or .store path. > > + */ > > + mutex_unlock(&q->sysfs_lock); > > elv_unregister_queue(q); > > + mutex_lock(&q->sysfs_lock); > > + } > > ioc_clear_queue(q); > > elevator_exit(q, q->elevator); > > } > > Hi Ming, > > I don't like this part of the patch. Consider the following call chain: > > queue_attr_store() -> elv_iosched_store() -> __elevator_change() -> > elevator_switch() -> elevator_switch_mq(). > > queue_attr_store() locks sysfs_lock to serialize sysfs attribute show > and store callbacks. So the above changes unlocks sysfs_lock from inside > such a callback function and hence breaks that serialization. I guess we have to break the serialization, because we can't hold q->sysfs_lock when deleting iosched kobjects. And we may re-check QUEUE_FLAG_REGISTERED after grabbing .sysfs_lock again, and stop to switch to new scheduler if queue is un-registered. > Can you > have a look at the alternative patch below? Sure. > > Thanks, > > Bart. > > > Subject: [PATCH] block: Fix lock inversion triggered during request queue removal > > Call blk_mq_unregister_dev() after having deleted q->kobj. Move > the kobject_uevent(q->mq_kobj, KOBJ_REMOVE) call from inside > blk_mq_unregister_dev() to its caller. > > --- > block/blk-mq-sysfs.c | 5 ++--- > block/blk-sysfs.c | 19 +++---------------- > block/elevator.c | 2 -- > 3 files changed, 5 insertions(+), 21 deletions(-) > > diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c > index d6e1a9bd7131..0ec968009791 100644 > --- a/block/blk-mq-sysfs.c > +++ b/block/blk-mq-sysfs.c > @@ -270,16 +270,15 @@ void blk_mq_unregister_dev(struct device *dev, struct request_queue *q) > struct blk_mq_hw_ctx *hctx; > int i; > > - lockdep_assert_held(&q->sysfs_lock); > - > queue_for_each_hw_ctx(q, hctx, i) > blk_mq_unregister_hctx(hctx); > > - kobject_uevent(q->mq_kobj, KOBJ_REMOVE); > kobject_del(q->mq_kobj); > kobject_put(&dev->kobj); > > + mutex_lock(&q->sysfs_lock); > q->mq_sysfs_init_done = false; > + mutex_unlock(&q->sysfs_lock); > } > > void blk_mq_hctx_kobj_init(struct blk_mq_hw_ctx *hctx) > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c > index 977c659dcd18..e6f8cd99aded 100644 > --- a/block/blk-sysfs.c > +++ b/block/blk-sysfs.c > @@ -1029,31 +1029,18 @@ void blk_unregister_queue(struct gendisk *disk) > if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags)) > return; > > - /* > - * Since sysfs_remove_dir() prevents adding new directory entries > - * before removal of existing entries starts, protect against > - * concurrent elv_iosched_store() calls. > - */ > - mutex_lock(&q->sysfs_lock); > - > blk_queue_flag_clear(QUEUE_FLAG_REGISTERED, q); .sysfs_lock has to be held for clearing 'REGISTERED', we have to wait for concurrent store to 'queue/scheduler' before un-registering queue. > > - /* > - * Remove the sysfs attributes before unregistering the queue data > - * structures that can be modified through sysfs. > - */ > if (queue_is_mq(q)) > - blk_mq_unregister_dev(disk_to_dev(disk), q); > - mutex_unlock(&q->sysfs_lock); > - > + kobject_uevent(q->mq_kobj, KOBJ_REMOVE); Could you explain why you move the above line here? > kobject_uevent(&q->kobj, KOBJ_REMOVE); > kobject_del(&q->kobj); > blk_trace_remove_sysfs(disk_to_dev(disk)); > > - mutex_lock(&q->sysfs_lock); > + if (queue_is_mq(q)) > + blk_mq_unregister_dev(disk_to_dev(disk), q); > if (q->elevator) > elv_unregister_queue(q); > - mutex_unlock(&q->sysfs_lock); > > kobject_put(&disk_to_dev(disk)->kobj); > } > diff --git a/block/elevator.c b/block/elevator.c > index 2f17d66d0e61..128e7cf032e1 100644 > --- a/block/elevator.c > +++ b/block/elevator.c > @@ -495,8 +495,6 @@ int elv_register_queue(struct request_queue *q) > > void elv_unregister_queue(struct request_queue *q) > { > - lockdep_assert_held(&q->sysfs_lock); > - > if (q) { > struct elevator_queue *e = q->elevator; elv_unregister_queue() is still called in elevator_switch_mq() with .sysfs_lock held, then potential AB-BA lock still exists. The same issue exists on blk_mq_sysfs_unregister() too. Thanks, Ming