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Tue, 22 Apr 2025 04:13:30 -0400 X-MC-Unique: GF6PEemsOSSMgWl7xRmwVA-1 X-Mimecast-MFC-AGG-ID: GF6PEemsOSSMgWl7xRmwVA_1745309609 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E9A931800368; Tue, 22 Apr 2025 08:13:28 +0000 (UTC) Received: from fedora (unknown [10.72.116.157]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 654E9180047F; Tue, 22 Apr 2025 08:13:23 +0000 (UTC) Date: Tue, 22 Apr 2025 16:13:19 +0800 From: Ming Lei To: Nilay Shroff Cc: Jens Axboe , linux-block@vger.kernel.org, Shinichiro Kawasaki , Thomas =?iso-8859-1?Q?Hellstr=F6m?= , Christoph Hellwig Subject: Re: [PATCH V2 20/20] block: move wbt_enable_default() out of queue freezing from sched ->exit() Message-ID: References: <20250418163708.442085-1-ming.lei@redhat.com> <20250418163708.442085-21-ming.lei@redhat.com> <261d7b81-e611-47f4-ad55-6f7716c278c7@linux.ibm.com> <286f9d0e-b782-4062-b0eb-cba6fa81e388@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <286f9d0e-b782-4062-b0eb-cba6fa81e388@linux.ibm.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On Tue, Apr 22, 2025 at 11:44:59AM +0530, Nilay Shroff wrote: > > > On 4/21/25 12:57 PM, Ming Lei wrote: > > On Sat, Apr 19, 2025 at 08:09:04PM +0530, Nilay Shroff wrote: > >> > >> > >> On 4/18/25 10:07 PM, Ming Lei wrote: > >>> scheduler's ->exit() is called with queue frozen and elevator lock is held, and > >>> wbt_enable_default() can't be called with queue frozen, otherwise the > >>> following lockdep warning is triggered: > >>> > >>> #6 (&q->rq_qos_mutex){+.+.}-{4:4}: > >>> #5 (&eq->sysfs_lock){+.+.}-{4:4}: > >>> #4 (&q->elevator_lock){+.+.}-{4:4}: > >>> #3 (&q->q_usage_counter(io)#3){++++}-{0:0}: > >>> #2 (fs_reclaim){+.+.}-{0:0}: > >>> #1 (&sb->s_type->i_mutex_key#3){+.+.}-{4:4}: > >>> #0 (&q->debugfs_mutex){+.+.}-{4:4}: > >>> > >>> Fix the issue by moving wbt_enable_default() out of bfq's exit(), and > >>> call it from elevator_change_done(). > >>> > >>> Signed-off-by: Ming Lei > >>> --- > >>> block/bfq-iosched.c | 2 +- > >>> block/elevator.c | 5 +++++ > >>> block/elevator.h | 1 + > >>> 3 files changed, 7 insertions(+), 1 deletion(-) > >>> > >>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c > >>> index 40e4106a71e7..310ce1d8c41e 100644 > >>> --- a/block/bfq-iosched.c > >>> +++ b/block/bfq-iosched.c > >>> @@ -7211,7 +7211,7 @@ static void bfq_exit_queue(struct elevator_queue *e) > >>> > >>> blk_stat_disable_accounting(bfqd->queue); > >>> blk_queue_flag_clear(QUEUE_FLAG_DISABLE_WBT, bfqd->queue); > >>> - wbt_enable_default(bfqd->queue->disk); > >>> + set_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT, &e->flags); > >>> > >>> kfree(bfqd); > >>> } > >>> diff --git a/block/elevator.c b/block/elevator.c > >>> index 8652fe45a2db..378553fce5d8 100644 > >>> --- a/block/elevator.c > >>> +++ b/block/elevator.c > >>> @@ -687,8 +687,13 @@ int elevator_change_done(struct request_queue *q, struct elv_change_ctx *ctx) > >>> int ret = 0; > >>> > >>> if (ctx->old) { > >>> + bool enable_wbt = test_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT, > >>> + &ctx->old->flags); > >>> + > >>> elv_unregister_queue(q, ctx->old); > >>> kobject_put(&ctx->old->kobj); > >>> + if (enable_wbt) > >>> + wbt_enable_default(q->disk); > >>> } > >>> if (ctx->new) { > >>> ret = elv_register_queue(q, ctx->new, ctx->uevent); > >>> diff --git a/block/elevator.h b/block/elevator.h > >>> index 486be0690499..b14c611c74b6 100644 > >>> --- a/block/elevator.h > >>> +++ b/block/elevator.h > >>> @@ -122,6 +122,7 @@ struct elevator_queue > >>> > >>> #define ELEVATOR_FLAG_REGISTERED 0 > >>> #define ELEVATOR_FLAG_DYING 1 > >>> +#define ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT 2 > >>> > >>> /* Holding context data for changing elevator */ > >>> struct elv_change_ctx { > >> > >> It seems invoking wbt_enable_default from elevator_change_done could probably > >> still race with ioc_qos_write or queue_wb_lat_store. Both ioc_qos_write and > >> queue_wb_lat_store run with ->freeze_lock and ->elevator_lock protection. > > > > Actually wbt_enable_default() and wbt_init() needn't the above protection, > > especially since the patch 2/20 removes q->elevator use in > > wbt_enable_default(). > > > Yes agreed, and as I understand XXX_FLAG_DISABLE_WBT was earlier elevator_queue->flags > but now (with patch 2/20) it has been moved to request_queue->flags. As elevator_change_done > first puts elevator_queue object which would potentially releases/frees the elevator_queue > object. Next while we enable wbt (in elevator_change_done) we may not have access to the > elevator_queue object and so now we reference QUEUE_FLAG_DISABLE_WBT using request_queue->flags. > That's, I believe, the purpose of patch 2/20. > > However even with patch 2/20 change, both elevator_change_done and ioc_qos_write or > queue_wb_lat_store may run in parallel, isn't it? > > therad1: > blk_mq_update_nr_hw_queues > -> __blk_mq_update_nr_hw_queues > -> elevator_change_done > -> wbt_enable_default > -> wbt_init > -> wbt_update_limits Here wbt_update_limits() is called on un-attached `struct rq_wb` instance, which is just allocated from heap. > > therad2: > queue_wb_lat_store > -> wbt_set_min_lat > -> wbt_update_limits The above one is run on attached `struct rq_wb` instance. And there can only be one attached `struct rq_wb` instance, so the above race doesn't exist because attaching wbt to queue/disk is covered by `q->rq_qos_mutex`. Thanks, Ming