From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ej1-f49.google.com (mail-ej1-f49.google.com [209.85.218.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A28DA2ECD1B for ; Thu, 4 Dec 2025 19:03:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.218.49 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764874985; cv=none; b=hL8r9GsepGy7gNy5hj5rLJ8Dywp6Xtzm9Iyv4/ojJgdMCVb3/W8Czx61WluU8WD2TakMsQGafXSbUbiNfbVaTSX20pyM0tB+OwNWG4YNPVY/E+md3qjXKv7LxER4mD0c4+K6AA02Ld8XxORunEW2YN3zrrYKJ3uoeNS9/MfBgG8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764874985; c=relaxed/simple; bh=cu623s1bdOx+4VnSqnuzChiFjH0jvM64sJHrEcwAaHo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ivhuzBJCHG1Re+NJUlJYjSLOdVT8LvKkMyzlsUk+l6wwg0KdlXrD66s+mjtUKTqGfZNFloYMtSht0aFHxMox9xWgIoeuSD52b//CX1iKuPooQ9qQmG1TeSzu7sdHNDBmZ9WEu9siohljVr7kUNcoNdb2Uq65ASHKaDVoERUXIDc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=purestorage.com; spf=fail smtp.mailfrom=purestorage.com; dkim=pass (2048-bit key) header.d=purestorage.com header.i=@purestorage.com header.b=QcDH8dIX; arc=none smtp.client-ip=209.85.218.49 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=purestorage.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=purestorage.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=purestorage.com header.i=@purestorage.com header.b="QcDH8dIX" Received: by mail-ej1-f49.google.com with SMTP id a640c23a62f3a-b728a43e410so213829666b.1 for ; Thu, 04 Dec 2025 11:03:03 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=purestorage.com; s=google2022; t=1764874982; x=1765479782; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=HZNwwr56flO1xS1S9sF1L+dtFjYocKmny67IwW4Ohtk=; b=QcDH8dIX5elujs+UyNxu+vGUaejX6rZqM5bXjQqwTLYV35rYAe71FcgpuOBa19t8ab INDqN4Q7zteA/h4DR0cCIbyKkOWwvxiq/Gp0TzjeLoy8PzZQkhLqxvwMnGj4KzNKYhCK WE4KaocAQP5m4pc3zS58mEqxGmYF/7FJxFstBsSpn63Yqxxc9Vii7bWXaoUQWLuOZybN hN2PiFuOcV9ZIBvyVKLZg5gHEIceNZZsASddwFRbrRfdHqM9RoIv+JWcm4esrolN55Wt At14rHUAMHNi8evRWR45sARbi3r+1yed5g2SSlAOA3ddnAf+NTtiYFl8HEibmvpZVxTj D7MQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1764874982; x=1765479782; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=HZNwwr56flO1xS1S9sF1L+dtFjYocKmny67IwW4Ohtk=; b=t6jC2uVUCiITQsBVYBMVeItBtWGgIWeGxzsk+1Yp8Adj+N2zUj69zDhkW9za1SxRnq 0hSvKEbjZJTq3qZ+h6Ule7y2Sh5ZsXWNGzfC2StDxWJudsuJqe3857wa5nu1OvQKDOPi NcKOH4VmC119EFHQoPrOPKTeG2K0XAbItMKcepiOBpWEXHU6N2P3QlIgA70xl4V2i2iz DTDLxGndilpddbNIgiNYyQBpbmUy7BddEmNRHIbc+0jKU9S4JUtyznRGz0yW1lwAXsrE omapdncssCdGUg1wniC1uwkwffTkeP1YN/H964DO8KLbt72ZiVqHvTPryuBxE+2GOuM9 tUZg== X-Forwarded-Encrypted: i=1; AJvYcCUJHgxz/AE3YoXqbMLIPikvA1+oGWWblzOE6Yq0OdeoBCXBiHA5Njt3Pj+SqhyZ7D5Pc/lZAg2BcaKDnWM=@vger.kernel.org X-Gm-Message-State: AOJu0YyyxyGSIE3TdjFGmqFpHlao53WcEyj+0+Mi0J4/JEMb40PBNfUL EOOaOkpVCWxf4xZQgdAuq+eM562+pwLPgzaUchCEDlwjKYBR039vErTSsYg8dkpPpB4= X-Gm-Gg: ASbGncuR05/oozilHpnfndOLQLZ0S3acdy0j+RBXEbRn+UZaUQJGxoVsdW1meYlzzws PC1FVIvJr9TUyhHszMzdovIgw7GOGaFXMMmCWu04YsabyOvg32NqUMNpIoLFCt62YQ4BaHB6lC3 cH6SFEL8FKnUWET2YoF895cjBNA7pt7hTu5CW0Gw0ukKE0cR3FWC2E4NAoilvDjyh0xP4aroz7R cDsWrlpCfi4WUp8d7JsRkWcG9g15bVOFGWAbwpVHGH2ZXaNn2AUz1CmuvmVqGLseIGZofDsP3r3 1je03fd+1/6VI9cjBCgkvoqyPq9XzddjesN2FLhJUJccZ1Xos5ABVZfV4U/ecNkBzZHDndRyK6Y jYWwut9HjOxWB55I/QkCWOIGjwlH2JirwqcinHcg6jfuDti20o7knYc5T8Z1Acgv5BUDMs8NIEy MI7mckHCY6dZKELdj60XFH9h6Qt+fWu+SSn11MtA5ZmA== X-Google-Smtp-Source: AGHT+IH26241l6Q/yokZ7U8z1pSwtQVJnt6+RwJRUNGc30R9za5E0rovz0FeljZNndRz6VFmAa7/zA== X-Received: by 2002:a17:907:3cc2:b0:b3c:3c8e:189d with SMTP id a640c23a62f3a-b79dc73cce5mr759963766b.32.1764874981679; Thu, 04 Dec 2025 11:03:01 -0800 (PST) Received: from medusa.lab.kspace.sh ([208.88.152.253]) by smtp.googlemail.com with UTF8SMTPSA id a640c23a62f3a-b79f449ba82sm195018266b.22.2025.12.04.11.03.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Dec 2025 11:03:01 -0800 (PST) Date: Thu, 4 Dec 2025 11:02:58 -0800 From: Mohamed Khalfella To: Chaitanya Kulkarni , Christoph Hellwig , Jens Axboe , Keith Busch , Sagi Grimberg Cc: Casey Chen , Yuanyuan Zhong , Hannes Reinecke , Ming Lei , Waiman Long , Hillf Danton , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/1] block: Use RCU in blk_mq_[un]quiesce_tagset() instead of set->tag_list_lock Message-ID: <20251204190258.GA337106-mkhalfella@purestorage.com> References: <20251204181212.1484066-1-mkhalfella@purestorage.com> <20251204181212.1484066-2-mkhalfella@purestorage.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=us-ascii Content-Disposition: inline In-Reply-To: <20251204181212.1484066-2-mkhalfella@purestorage.com> On Thu 2025-12-04 10:11:53 -0800, Mohamed Khalfella wrote: > blk_mq_{add,del}_queue_tag_set() functions add and remove queues from > tagset, the functions make sure that tagset and queues are marked as > shared when two or more queues are attached to the same tagset. > Initially a tagset starts as unshared and when the number of added > queues reaches two, blk_mq_add_queue_tag_set() marks it as shared along > with all the queues attached to it. When the number of attached queues > drops to 1 blk_mq_del_queue_tag_set() need to mark both the tagset and > the remaining queues as unshared. > > Both functions need to freeze current queues in tagset before setting on > unsetting BLK_MQ_F_TAG_QUEUE_SHARED flag. While doing so, both functions > hold set->tag_list_lock mutex, which makes sense as we do not want > queues to be added or deleted in the process. This used to work fine > until commit 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") > made the nvme driver quiesce tagset instead of quiscing individual > queues. blk_mq_quiesce_tagset() does the job and quiesce the queues in > set->tag_list while holding set->tag_list_lock also. > > This results in deadlock between two threads with these stacktraces: > > __schedule+0x48e/0xed0 > schedule+0x5a/0xc0 > schedule_preempt_disabled+0x11/0x20 > __mutex_lock.constprop.0+0x3cc/0x760 > blk_mq_quiesce_tagset+0x26/0xd0 > nvme_dev_disable_locked+0x77/0x280 [nvme] > nvme_timeout+0x268/0x320 [nvme] > blk_mq_handle_expired+0x5d/0x90 > bt_iter+0x7e/0x90 > blk_mq_queue_tag_busy_iter+0x2b2/0x590 > ? __blk_mq_complete_request_remote+0x10/0x10 > ? __blk_mq_complete_request_remote+0x10/0x10 > blk_mq_timeout_work+0x15b/0x1a0 > process_one_work+0x133/0x2f0 > ? mod_delayed_work_on+0x90/0x90 > worker_thread+0x2ec/0x400 > ? mod_delayed_work_on+0x90/0x90 > kthread+0xe2/0x110 > ? kthread_complete_and_exit+0x20/0x20 > ret_from_fork+0x2d/0x50 > ? kthread_complete_and_exit+0x20/0x20 > ret_from_fork_asm+0x11/0x20 > > __schedule+0x48e/0xed0 > schedule+0x5a/0xc0 > blk_mq_freeze_queue_wait+0x62/0x90 > ? destroy_sched_domains_rcu+0x30/0x30 > blk_mq_exit_queue+0x151/0x180 > disk_release+0xe3/0xf0 > device_release+0x31/0x90 > kobject_put+0x6d/0x180 > nvme_scan_ns+0x858/0xc90 [nvme_core] > ? nvme_scan_work+0x281/0x560 [nvme_core] > nvme_scan_work+0x281/0x560 [nvme_core] > process_one_work+0x133/0x2f0 > ? mod_delayed_work_on+0x90/0x90 > worker_thread+0x2ec/0x400 > ? mod_delayed_work_on+0x90/0x90 > kthread+0xe2/0x110 > ? kthread_complete_and_exit+0x20/0x20 > ret_from_fork+0x2d/0x50 > ? kthread_complete_and_exit+0x20/0x20 > ret_from_fork_asm+0x11/0x20 > > The top stacktrace is showing nvme_timeout() called to handle nvme > command timeout. timeout handler is trying to disable the controller and > as a first step, it needs to blk_mq_quiesce_tagset() to tell blk-mq not > to call queue callback handlers. The thread is stuck waiting for > set->tag_list_lock as it tires to walk the queues in set->tag_list. > > The lock is held by the second thread in the bottom stack which is > waiting for one of queues to be frozen. The queue usage counter will > drop to zero after nvme_timeout() finishes, and this will not happen > because the thread will wait for this mutex forever. > > Given that [un]quescing queue is an operation that does not need to > sleep, update blk_mq_[un]quiesce_tagset() to use RCU instead of taking > set->tag_list_lock. Also update blk_mq_{add,del}_queue_tag_set() to use > RCU safe list operations. This should help avoid deadlock seen above. > > Signed-off-by: Mohamed Khalfella Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") Oops, this should be v3 and I also missed Fixes: tag above. > --- > block/blk-mq.c | 18 ++++++++++-------- > 1 file changed, 10 insertions(+), 8 deletions(-) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index d626d32f6e57..ceb176ac154b 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -335,12 +335,12 @@ void blk_mq_quiesce_tagset(struct blk_mq_tag_set *set) > { > struct request_queue *q; > > - mutex_lock(&set->tag_list_lock); > - list_for_each_entry(q, &set->tag_list, tag_set_list) { > + rcu_read_lock(); > + list_for_each_entry_rcu(q, &set->tag_list, tag_set_list) { > if (!blk_queue_skip_tagset_quiesce(q)) > blk_mq_quiesce_queue_nowait(q); > } > - mutex_unlock(&set->tag_list_lock); > + rcu_read_unlock(); > > blk_mq_wait_quiesce_done(set); > } > @@ -350,12 +350,12 @@ void blk_mq_unquiesce_tagset(struct blk_mq_tag_set *set) > { > struct request_queue *q; > > - mutex_lock(&set->tag_list_lock); > - list_for_each_entry(q, &set->tag_list, tag_set_list) { > + rcu_read_lock(); > + list_for_each_entry_rcu(q, &set->tag_list, tag_set_list) { > if (!blk_queue_skip_tagset_quiesce(q)) > blk_mq_unquiesce_queue(q); > } > - mutex_unlock(&set->tag_list_lock); > + rcu_read_unlock(); > } > EXPORT_SYMBOL_GPL(blk_mq_unquiesce_tagset); > > @@ -4294,7 +4294,7 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q) > struct blk_mq_tag_set *set = q->tag_set; > > mutex_lock(&set->tag_list_lock); > - list_del(&q->tag_set_list); > + list_del_rcu(&q->tag_set_list); > if (list_is_singular(&set->tag_list)) { > /* just transitioned to unshared */ > set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED; > @@ -4302,6 +4302,8 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q) > blk_mq_update_tag_set_shared(set, false); > } > mutex_unlock(&set->tag_list_lock); > + > + synchronize_rcu(); > INIT_LIST_HEAD(&q->tag_set_list); > } > > @@ -4321,7 +4323,7 @@ static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set, > } > if (set->flags & BLK_MQ_F_TAG_QUEUE_SHARED) > queue_set_hctx_shared(q, true); > - list_add_tail(&q->tag_set_list, &set->tag_list); > + list_add_tail_rcu(&q->tag_set_list, &set->tag_list); > > mutex_unlock(&set->tag_list_lock); > } > -- > 2.51.2 >