From patchwork Tue Aug 21 07:15:03 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "jianchao.wang" X-Patchwork-Id: 10571107 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id CBF29139B for ; Tue, 21 Aug 2018 07:13:40 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id B676429A2D for ; Tue, 21 Aug 2018 07:13:40 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id AA6F329CB8; Tue, 21 Aug 2018 07:13:40 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.0 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI, UNPARSEABLE_RELAY autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id ED4FD29A2D for ; Tue, 21 Aug 2018 07:13:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726792AbeHUKc3 (ORCPT ); Tue, 21 Aug 2018 06:32:29 -0400 Received: from userp2130.oracle.com ([156.151.31.86]:44292 "EHLO userp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726253AbeHUKc3 (ORCPT ); Tue, 21 Aug 2018 06:32:29 -0400 Received: from pps.filterd (userp2130.oracle.com [127.0.0.1]) by userp2130.oracle.com (8.16.0.22/8.16.0.22) with SMTP id w7L790cO070841; Tue, 21 Aug 2018 07:13:27 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=from : to : cc : subject : date : message-id : in-reply-to : references; s=corp-2018-07-02; bh=UF1/mlU5g2XiCBwjJOjli9yb6K7NMgakc0k5V25svDw=; b=r51EC270IinIPTD/qwR3V5R2jNj5zJu7ZMKEaEC9qn5lS5skw26+sNNi1pwIyKJLSD7t 9UM4Ch7qpVSh3EI+XPVCw5mCLWmsTe3zmk0t3Ayn3J2ubbyegzvMtT4NruwPfMFbV/wY xCGtoCl6VBCsp5FDb6DB8sBO8jCO7grzfOQNrPI7ibPxIR5mljUNWy8aindkZ/zYnuKp sw7ExRTB1cBvkgX96FXcO9ZIb2xNuVA1tDOYv+o35+U9jz29c5JjwnV6sTNUffsTWwuP q/3lcT1YaVmfxW77qtwhGzhLDGDSYY9UuUmYW2qG52GjT3KwCu4kRSbY/YylMCSVtG9P PQ== Received: from userv0021.oracle.com (userv0021.oracle.com [156.151.31.71]) by userp2130.oracle.com with ESMTP id 2kxavtjjx2-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 21 Aug 2018 07:13:27 +0000 Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by userv0021.oracle.com (8.14.4/8.14.4) with ESMTP id w7L7DQlb027736 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 21 Aug 2018 07:13:27 GMT Received: from abhmp0015.oracle.com (abhmp0015.oracle.com [141.146.116.21]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id w7L7DQbM021326; Tue, 21 Aug 2018 07:13:26 GMT Received: from will-ThinkCentre-M910s.cn.oracle.com (/10.182.70.254) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 21 Aug 2018 00:13:26 -0700 From: Jianchao Wang To: axboe@kernel.dk Cc: tom.leiming@gmail.com, bart.vanassche@wdc.com, keith.busch@linux.intel.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH V4 1/2] blk-mq: init hctx sched after update ctx and hctx mapping Date: Tue, 21 Aug 2018 15:15:03 +0800 Message-Id: <1534835704-3607-2-git-send-email-jianchao.w.wang@oracle.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1534835704-3607-1-git-send-email-jianchao.w.wang@oracle.com> References: <1534835704-3607-1-git-send-email-jianchao.w.wang@oracle.com> X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=8991 signatures=668707 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=3 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1808210076 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Currently, when update nr_hw_queues, IO scheduler's init_hctx will be invoked before the mapping between ctx and hctx is adapted correctly by blk_mq_map_swqueue. The IO scheduler init_hctx (kyber) may depend on this mapping and get wrong result and panic finally. A simply way to fix this is that switch the IO scheduler to 'none' before update the nr_hw_queues, and then switch it back after update nr_hw_queues. blk_mq_sched_init_/exit_hctx are removed due to nobody use them any more. Signed-off-by: Jianchao Wang --- block/blk-mq-sched.c | 44 ------------------------- block/blk-mq-sched.h | 5 --- block/blk-mq.c | 92 +++++++++++++++++++++++++++++++++++++++++++++++----- block/blk.h | 2 ++ block/elevator.c | 20 +++++++----- 5 files changed, 98 insertions(+), 65 deletions(-) diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index cf9c66c..29bfe80 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -462,50 +462,6 @@ static void blk_mq_sched_tags_teardown(struct request_queue *q) blk_mq_sched_free_tags(set, hctx, i); } -int blk_mq_sched_init_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx, - unsigned int hctx_idx) -{ - struct elevator_queue *e = q->elevator; - int ret; - - if (!e) - return 0; - - ret = blk_mq_sched_alloc_tags(q, hctx, hctx_idx); - if (ret) - return ret; - - if (e->type->ops.mq.init_hctx) { - ret = e->type->ops.mq.init_hctx(hctx, hctx_idx); - if (ret) { - blk_mq_sched_free_tags(q->tag_set, hctx, hctx_idx); - return ret; - } - } - - blk_mq_debugfs_register_sched_hctx(q, hctx); - - return 0; -} - -void blk_mq_sched_exit_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx, - unsigned int hctx_idx) -{ - struct elevator_queue *e = q->elevator; - - if (!e) - return; - - blk_mq_debugfs_unregister_sched_hctx(hctx); - - if (e->type->ops.mq.exit_hctx && hctx->sched_data) { - e->type->ops.mq.exit_hctx(hctx, hctx_idx); - hctx->sched_data = NULL; - } - - blk_mq_sched_free_tags(q->tag_set, hctx, hctx_idx); -} - int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e) { struct blk_mq_hw_ctx *hctx; diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h index 0cb8f93..4e028ee 100644 --- a/block/blk-mq-sched.h +++ b/block/blk-mq-sched.h @@ -28,11 +28,6 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx); int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e); void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e); -int blk_mq_sched_init_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx, - unsigned int hctx_idx); -void blk_mq_sched_exit_hctx(struct request_queue *q, struct blk_mq_hw_ctx *hctx, - unsigned int hctx_idx); - static inline bool blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio) { diff --git a/block/blk-mq.c b/block/blk-mq.c index 5efd789..9c8c8c7 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2147,8 +2147,6 @@ static void blk_mq_exit_hctx(struct request_queue *q, if (set->ops->exit_request) set->ops->exit_request(set, hctx->fq->flush_rq, hctx_idx); - blk_mq_sched_exit_hctx(q, hctx, hctx_idx); - if (set->ops->exit_hctx) set->ops->exit_hctx(hctx, hctx_idx); @@ -2216,12 +2214,9 @@ static int blk_mq_init_hctx(struct request_queue *q, set->ops->init_hctx(hctx, set->driver_data, hctx_idx)) goto free_bitmap; - if (blk_mq_sched_init_hctx(q, hctx, hctx_idx)) - goto exit_hctx; - hctx->fq = blk_alloc_flush_queue(q, hctx->numa_node, set->cmd_size); if (!hctx->fq) - goto sched_exit_hctx; + goto exit_hctx; if (blk_mq_init_request(set, hctx->fq->flush_rq, hctx_idx, node)) goto free_fq; @@ -2235,8 +2230,6 @@ static int blk_mq_init_hctx(struct request_queue *q, free_fq: kfree(hctx->fq); - sched_exit_hctx: - blk_mq_sched_exit_hctx(q, hctx, hctx_idx); exit_hctx: if (set->ops->exit_hctx) set->ops->exit_hctx(hctx, hctx_idx); @@ -2898,10 +2891,81 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr) return ret; } +/* + * request_queue and elevator_type pair. + * It is just used by __blk_mq_update_nr_hw_queues to cache + * the elevator_type associated with a request_queue. + */ +struct blk_mq_qe_pair { + struct list_head node; + struct request_queue *q; + struct elevator_type *type; +}; + +/* + * Cache the elevator_type in qe pair list and switch the + * io scheduler to 'none' + */ +static bool blk_mq_elv_switch_none(struct list_head *head, + struct request_queue *q) +{ + struct blk_mq_qe_pair *qe; + + if (!q->elevator) + return true; + + qe = kmalloc(sizeof(*qe), GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY); + if (!qe) + return false; + + INIT_LIST_HEAD(&qe->node); + qe->q = q; + qe->type = q->elevator->type; + list_add(&qe->node, head); + + mutex_lock(&q->sysfs_lock); + /* + * After elevator_switch_mq, the previous elevator_queue will be + * released by elevator_release. The reference of the io scheduler + * module get by elevator_get will also be put. So we need to get + * a reference of the io scheduler module here to prevent it to be + * removed. + */ + __module_get(qe->type->elevator_owner); + elevator_switch_mq(q, NULL); + mutex_unlock(&q->sysfs_lock); + + return true; +} + +static void blk_mq_elv_switch_back(struct list_head *head, + struct request_queue *q) +{ + struct blk_mq_qe_pair *qe; + struct elevator_type *t = NULL; + + list_for_each_entry(qe, head, node) + if (qe->q == q) { + t = qe->type; + break; + } + + if (!t) + return; + + list_del(&qe->node); + kfree(qe); + + mutex_lock(&q->sysfs_lock); + elevator_switch_mq(q, t); + mutex_unlock(&q->sysfs_lock); +} + static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues) { struct request_queue *q; + LIST_HEAD(head); lockdep_assert_held(&set->tag_list_lock); @@ -2912,6 +2976,14 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, list_for_each_entry(q, &set->tag_list, tag_set_list) blk_mq_freeze_queue(q); + /* + * Switch IO scheduler to 'none', cleaning up the data associated + * with the previous scheduler. We will switch back once we are done + * updating the new sw to hw queue mappings. + */ + list_for_each_entry(q, &set->tag_list, tag_set_list) + if (!blk_mq_elv_switch_none(&head, q)) + goto switch_back; set->nr_hw_queues = nr_hw_queues; blk_mq_update_queue_map(set); @@ -2920,6 +2992,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, blk_mq_queue_reinit(q); } +switch_back: + list_for_each_entry(q, &set->tag_list, tag_set_list) + blk_mq_elv_switch_back(&head, q); + list_for_each_entry(q, &set->tag_list, tag_set_list) blk_mq_unfreeze_queue(q); } diff --git a/block/blk.h b/block/blk.h index d4d67e9..0c9bc8d 100644 --- a/block/blk.h +++ b/block/blk.h @@ -234,6 +234,8 @@ static inline void elv_deactivate_rq(struct request_queue *q, struct request *rq int elevator_init(struct request_queue *); int elevator_init_mq(struct request_queue *q); +int elevator_switch_mq(struct request_queue *q, + struct elevator_type *new_e); void elevator_exit(struct request_queue *, struct elevator_queue *); int elv_register_queue(struct request_queue *q); void elv_unregister_queue(struct request_queue *q); diff --git a/block/elevator.c b/block/elevator.c index fa828b5..5ea6e7d 100644 --- a/block/elevator.c +++ b/block/elevator.c @@ -933,16 +933,13 @@ void elv_unregister(struct elevator_type *e) } EXPORT_SYMBOL_GPL(elv_unregister); -static int elevator_switch_mq(struct request_queue *q, +int elevator_switch_mq(struct request_queue *q, struct elevator_type *new_e) { int ret; lockdep_assert_held(&q->sysfs_lock); - blk_mq_freeze_queue(q); - blk_mq_quiesce_queue(q); - if (q->elevator) { if (q->elevator->registered) elv_unregister_queue(q); @@ -968,8 +965,6 @@ static int elevator_switch_mq(struct request_queue *q, blk_add_trace_msg(q, "elv switch: none"); out: - blk_mq_unquiesce_queue(q); - blk_mq_unfreeze_queue(q); return ret; } @@ -1021,8 +1016,17 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e) lockdep_assert_held(&q->sysfs_lock); - if (q->mq_ops) - return elevator_switch_mq(q, new_e); + if (q->mq_ops) { + blk_mq_freeze_queue(q); + blk_mq_quiesce_queue(q); + + err = elevator_switch_mq(q, new_e); + + blk_mq_unquiesce_queue(q); + blk_mq_unfreeze_queue(q); + + return err; + } /* * Turn on BYPASS and drain all requests w/ elevator private data. From patchwork Tue Aug 21 07:15:04 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "jianchao.wang" X-Patchwork-Id: 10571109 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id D6B99921 for ; Tue, 21 Aug 2018 07:14:34 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id C1D9429CB7 for ; Tue, 21 Aug 2018 07:14:34 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id B62B929CB9; Tue, 21 Aug 2018 07:14:34 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.0 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI, UNPARSEABLE_RELAY autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 562A229CB7 for ; Tue, 21 Aug 2018 07:14:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726315AbeHUKda (ORCPT ); Tue, 21 Aug 2018 06:33:30 -0400 Received: from userp2120.oracle.com ([156.151.31.85]:45968 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726253AbeHUKda (ORCPT ); Tue, 21 Aug 2018 06:33:30 -0400 Received: from pps.filterd (userp2120.oracle.com [127.0.0.1]) by userp2120.oracle.com (8.16.0.22/8.16.0.22) with SMTP id w7L78xel056050; Tue, 21 Aug 2018 07:13:29 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=from : to : cc : subject : date : message-id : in-reply-to : references; s=corp-2018-07-02; bh=znaTFbEDOmUEg8O+aRKk96aenhOx02Xvk1T4VRvWen4=; b=bJ5wC+CPPvArS3RBMTMh8iTiF0U6RpciJw9yd+mWWxi9wxIdyC/UIxoYDDZW97IZaw5/ W2AkX9NL3BRy2wfEZIy39GtTlbSQ/chsiPOTpn5D02r18hwXG55ku6dOTgOcqpb8VljJ 9bCfIGVoUWkYdmdn/8AtcU2Iywp0JS1twv6nmEglMeDlIsMxQ0MICC4kMRiWw/SkLRU2 DqsebSbqbTLcYnyB+rr8X4WL/keRczlNHxHz5KALTrkUIx3zG/q5D8MdASj4LH4sYDxw tOb3PYVs8OcPbZa2QnY/XNtpv6VF3iuupgw8D3a9LtAmwLYhQbP18dzn1ag+Z4C4cx9q xQ== Received: from userv0022.oracle.com (userv0022.oracle.com [156.151.31.74]) by userp2120.oracle.com with ESMTP id 2kxc3qjfu3-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 21 Aug 2018 07:13:29 +0000 Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by userv0022.oracle.com (8.14.4/8.14.4) with ESMTP id w7L7DSqY010418 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 21 Aug 2018 07:13:29 GMT Received: from abhmp0015.oracle.com (abhmp0015.oracle.com [141.146.116.21]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id w7L7DS3N021343; Tue, 21 Aug 2018 07:13:28 GMT Received: from will-ThinkCentre-M910s.cn.oracle.com (/10.182.70.254) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 21 Aug 2018 00:13:28 -0700 From: Jianchao Wang To: axboe@kernel.dk Cc: tom.leiming@gmail.com, bart.vanassche@wdc.com, keith.busch@linux.intel.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH V4 2/2] blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter Date: Tue, 21 Aug 2018 15:15:04 +0800 Message-Id: <1534835704-3607-3-git-send-email-jianchao.w.wang@oracle.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1534835704-3607-1-git-send-email-jianchao.w.wang@oracle.com> References: <1534835704-3607-1-git-send-email-jianchao.w.wang@oracle.com> X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=8991 signatures=668707 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=1 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1808210076 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP For blk-mq, part_in_flight/rw will invoke blk_mq_in_flight/rw to account the inflight requests. It will access the queue_hw_ctx and nr_hw_queues w/o any protection. When updating nr_hw_queues and blk_mq_in_flight/rw occur concurrently, panic comes up. Before update nr_hw_queues, the q will be frozen. So we could use q_usage_counter to avoid the race. percpu_ref_is_zero is used here so that we will not miss any in-flight request. The access to nr_hw_queues and queue_hw_ctx in blk_mq_queue_tag_busy_iter are under rcu critical section, __blk_mq_update_nr_hw_queues could use synchronize_rcu to ensure the zeroed q_usage_counter to be globally visible. Signed-off-by: Jianchao Wang Reviewed-by: Ming Lei --- block/blk-mq-tag.c | 14 +++++++++++++- block/blk-mq.c | 4 ++++ 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index c0c4e63..8c5cc11 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -320,6 +320,18 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn, struct blk_mq_hw_ctx *hctx; int i; + /* + * __blk_mq_update_nr_hw_queues will update the nr_hw_queues and + * queue_hw_ctx after freeze the queue. So we could use q_usage_counter + * to avoid race with it. __blk_mq_update_nr_hw_queues will users + * synchronize_rcu to ensure all of the users go out of the critical + * section below and see zeroed q_usage_counter. + */ + rcu_read_lock(); + if (percpu_ref_is_zero(&q->q_usage_counter)) { + rcu_read_unlock(); + return; + } queue_for_each_hw_ctx(q, hctx, i) { struct blk_mq_tags *tags = hctx->tags; @@ -335,7 +347,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn, bt_for_each(hctx, &tags->breserved_tags, fn, priv, true); bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false); } - + rcu_read_unlock(); } static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth, diff --git a/block/blk-mq.c b/block/blk-mq.c index 9c8c8c7..81cb84b 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2977,6 +2977,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, list_for_each_entry(q, &set->tag_list, tag_set_list) blk_mq_freeze_queue(q); /* + * Sync with blk_mq_queue_tag_busy_iter. + */ + synchronize_rcu(); + /* * Switch IO scheduler to 'none', cleaning up the data associated * with the previous scheduler. We will switch back once we are done * updating the new sw to hw queue mappings.