From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6A10E319611 for ; Wed, 15 Oct 2025 09:27:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760520480; cv=none; b=hLJmFEmVQ8TDqg+SVdy9xHNbjds38/LEa1cnFGb8C68Ie+aYyniBN0DbSTHqcqzDrMktUfTwImjx37994q06RMvtA3C1h/YvK810itXwo7PFFKJCCU3+O5XK/RyfCrWTAxcdW6xaudVGrnBJ93sc5yEJOSbddz3ywNesIftpnJA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760520480; c=relaxed/simple; bh=sPEg7pbtP1zrTtIJbq5Abj7JdtBqRqEL/3AmT/ihs9E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=URjwqRVefA5/oqYWy1kFkLk+Eq/7YMCSEqld9PvmaUyHwcSwuEnun9pOU3I2R/a0sBYAZ0JlOUHsItx3vGglvQM75KAb4XvMcezS1ranxWIVWUaXvho0X3gQG/WxQZqrp/RyHlxNFEKt8Vi/WIQa1B5tiQffIogblGTsJ852Yp4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=QsvUljDs; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="QsvUljDs" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1760520477; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8FvuC0nI11pMcake8zb9ZQ7cv8pD2b0TiH26/DUH8Ho=; b=QsvUljDseVEkZm+Cs1Wc42GLisee5i/MNyjA+2DO+yw4lDq/C7oWOUp/0piCbJh4UenuAw 8ftilas3/eA4SgQ3pidd/nTaWwUcHXa9378zLMXrDzMG2JZVnYwyjbV1nLLAs/XQQyrfCi VdKlZSGAj5iygm309nOz9cOdf3NT4SA= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-73-jFU6B8r8NgieqhpcBtUZ-Q-1; Wed, 15 Oct 2025 05:27:53 -0400 X-MC-Unique: jFU6B8r8NgieqhpcBtUZ-Q-1 X-Mimecast-MFC-AGG-ID: jFU6B8r8NgieqhpcBtUZ-Q_1760520471 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 154F41800245; Wed, 15 Oct 2025 09:27:50 +0000 (UTC) Received: from fedora (unknown [10.72.120.29]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 47DA618003FC; Wed, 15 Oct 2025 09:27:40 +0000 (UTC) Date: Wed, 15 Oct 2025 17:27:35 +0800 From: Ming Lei To: Nilay Shroff Cc: Yu Kuai , Yu Kuai , tj@kernel.org, josef@toxicpanda.com, axboe@kernel.dk, cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, yukuai1@huaweicloud.com, yi.zhang@huawei.com, yangerkun@huawei.com, johnny.chenyi@huawei.com Subject: Re: [PATCH 0/4] blk-rq-qos: fix possible deadlock Message-ID: References: <20251014022149.947800-1-yukuai3@huawei.com> <4b8aab6f-f341-49af-9ccb-d592e1a40fe5@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4b8aab6f-f341-49af-9ccb-d592e1a40fe5@linux.ibm.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 On Wed, Oct 15, 2025 at 10:46:51AM +0530, Nilay Shroff wrote: > > > On 10/15/25 7:12 AM, Ming Lei wrote: > > On Tue, Oct 14, 2025 at 07:14:16PM +0800, Yu Kuai wrote: > >> Hi, > >> > >> 在 2025/10/14 18:58, Nilay Shroff 写道: > >>> > >>> On 10/14/25 7:51 AM, Yu Kuai wrote: > >>>> Currently rq-qos debugfs entries is created from rq_qos_add(), while > >>>> rq_qos_add() requires queue to be freezed. This can deadlock because > >>>> > >>>> creating new entries can trigger fs reclaim. > >>>> > >>>> Fix this problem by delaying creating rq-qos debugfs entries until > >>>> it's initialization is complete. > >>>> > >>>> - For wbt, it can be initialized by default of by blk-sysfs, fix it by > >>>> delaying after wbt_init(); > >>>> - For other policies, they can only be initialized by blkg configuration, > >>>> fix it by delaying to blkg_conf_end(); > >>>> > >>>> Noted this set is cooked on the top of my other thread: > >>>> https://lore.kernel.org/all/20251010091446.3048529-1-yukuai@kernel.org/ > >>>> > >>>> And the deadlock can be reporduced with above thead, by running blktests > >>>> throtl/001 with wbt enabled by default. While the deadlock is really a > >>>> long term problem. > >>>> > >>> While freezing the queue we also mark GFP_NOIO scope, so doesn't that > >>> help avoid fs-reclaim? Or maybe if you can share the lockdep splat > >>> encountered running throtl/001? > >> > >> Yes, we can avoid fs-reclaim if queue is freezing, however, > >> because debugfs is a generic file system, and we can't avoid fs reclaim from > >> all context. There is still > >> > >> Following is the log with above set and wbt enabled by default, the set acquire > >> lock order by: > >> > >> freeze queue -> elevator lock -> rq_qos_mutex -> blkcg_mutex > >> > >> However, fs-reclaim from other context cause the deadlock report. > >> > >> > >> [ 45.632372][ T531] ====================================================== > >> [ 45.633734][ T531] WARNING: possible circular locking dependency detected > >> [ 45.635062][ T531] 6.17.0-gfd4a560a0864-dirty #30 Not tainted > >> [ 45.636220][ T531] ------------------------------------------------------ > >> [ 45.637587][ T531] check/531 is trying to acquire lock: > >> [ 45.638626][ T531] ffff9473884382b0 (&q->rq_qos_mutex){+.+.}-{4:4}, at: blkg_ > >> conf_start+0x116/0x190 > >> [ 45.640416][ T531] > >> [ 45.640416][ T531] but task is already holding lock: > >> [ 45.641828][ T531] ffff9473884385d8 (&q->elevator_lock){+.+.}-{4:4}, at: blkg > >> _conf_start+0x108/0x190 > >> [ 45.643322][ T531] > >> [ 45.643322][ T531] which lock already depends on the new lock. > >> [ 45.643322][ T531] > >> [ 45.644862][ T531] > >> [ 45.644862][ T531] the existing dependency chain (in reverse order) is: > >> [ 45.646046][ T531] > >> [ 45.646046][ T531] -> #5 (&q->elevator_lock){+.+.}-{4:4}: > >> [ 45.647052][ T531] __mutex_lock+0xd3/0x8d0 > >> [ 45.647716][ T531] blkg_conf_start+0x108/0x190 > >> [ 45.648395][ T531] tg_set_limit+0x74/0x300 > >> [ 45.649046][ T531] kernfs_fop_write_iter+0x14a/0x210 > >> [ 45.649813][ T531] vfs_write+0x29e/0x550 > >> [ 45.650413][ T531] ksys_write+0x74/0xf0 > >> [ 45.651032][ T531] do_syscall_64+0xbb/0x380 > >> [ 45.651730][ T531] entry_SYSCALL_64_after_hwframe+0x77/0x7f > > > > Not sure why elevator lock is grabbed in throttle code, which looks a elevator lock > > misuse, what does the elevator try to protect here? > > > > The comment log doesn't explain the usage too: > > > Lets go back to the history: > The ->elevator_lock was first added in the wbt code path under this commit > 245618f8e45f ("block: protect wbt_lat_usec using q->elevator_lock"). It was > introduced to protect the wbt latency and state updates which could be > simultaneously accessed from elevator switch path and from sysfs write method > (queue_wb_lat_store()) as well as from cgroup (ioc_qos_write()). > > Later above change caused a lockdep splat and then we updated the code > to fix locking order between ->freeze_lock, ->elevator_lock and ->rq_qos_mutex > and that was implemented in this commit 9730763f4756 ("block: correct locking > order for protecting blk-wbt parameters"). With this change we set the > locking order as follows: > ->freeze_lock ->elevator_lock ->rq_qos_mutex > > Then later on under this commit 78c271344b6f ("block: move wbt_enable_default() > out of queue freezing from sched ->exit()") we moved the wbt latency/stat > update code out of the ->freeze_lock and ->elevator_lock from elevator switch > path. So essentially with this commit now in theory we don't need to acquire > ->elevator_lock while updating wbt latency/stat values. In fact, we also removed > ->elevator_lock from queue_wb_lat_store() in this commit but I think we missed > to remove ->elevator_lock from cgroup (ioc_qos_write()). Nice looking back! I will cook one patch to remove it from ioc_qos_write(). > > > > > I think it is still order issue between queue freeze and q->rq_qos_mutex > > first, which need to be solved first. > > > So yes we should first target to get rid off the use of ->elevator_lock > from ioc_qos_write(). Later we can decide on locking order between > ->freeze_lock, ->rq_qos_mutex and ->debugfs_mutex. Yu Kuai is working on ordering queue freeze and ->rq_qos_mutex. Thanks, Ming