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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 2657DCCD19A for ; Mon, 17 Nov 2025 02:30:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To: Content-Transfer-Encoding:Content-Type:MIME-Version:References:Message-ID: Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=ZAt5O2OGCvrxi43qiiw7KlLubL52Moiy7Jw1EGFHlqI=; b=nVN/cS0YzXewiAkjxSZGkJVZPu Wpad3ZHSXrUpA2azJTPY3nNrx/o+ZXYP75vgMS/yaIzso417fnvfhSHRLCfAwR/rGbhphc/yXRfLE afaWq24FaX9Qky5sRj1PSSismrb8XuUkoVhfLZAIYdw8w6deozYxhpvkOOgp1LFBucaNOXAniAZ2l AcLUo3LdtPzyfTlgC31pfpG9jmnO7NTVaz+dRYneIddOCXSzKYmtZ5CGb/uptU8SetI3RwuINCeR9 6YKqeqkpD5sAcroqkHk6bcsC0an6lk8E//MbnHho/J97iLkkb7cTP/F7E4wbO5INb+s113XFRumVI TEfLZzCg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vKp14-0000000FJAU-1gTR; Mon, 17 Nov 2025 02:30:54 +0000 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vKp12-0000000FJA7-0Ldm for linux-nvme@lists.infradead.org; Mon, 17 Nov 2025 02:30:53 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1763346650; 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=ZAt5O2OGCvrxi43qiiw7KlLubL52Moiy7Jw1EGFHlqI=; b=ikv/qMOB236jESMBn/lQc1Z6VXYiaHs1eOI4hh+ELt34qeIm7pKflbfXNojknScaQ3raU2 WN6PD9IHCrWg1sjJKGBsuXZ+ZwyqBQ1+SgTNC8uRG8/KuPl9ki2yTQe40ArGTmlK5QN7FU +F1JK3MugpvPU5j/6pPijzcda15/bJE= Received: from mx-prod-mc-08.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-155-03LoMAaWNH6HgnL1Df_Huw-1; Sun, 16 Nov 2025 21:30:46 -0500 X-MC-Unique: 03LoMAaWNH6HgnL1Df_Huw-1 X-Mimecast-MFC-AGG-ID: 03LoMAaWNH6HgnL1Df_Huw_1763346645 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (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 5E7BA1800342; Mon, 17 Nov 2025 02:30:44 +0000 (UTC) Received: from fedora (unknown [10.72.116.55]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3BEB93002D0D; Mon, 17 Nov 2025 02:30:35 +0000 (UTC) Date: Mon, 17 Nov 2025 10:30:31 +0800 From: Ming Lei To: Mohamed Khalfella Cc: Jens Axboe , Keith Busch , Sagi Grimberg , Chaitanya Kulkarni , Casey Chen , Vikas Manocha , Yuanyuan Zhong , Hannes Reinecke , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] nvme: Convert tag_list mutex to rwsemaphore to avoid deadlock Message-ID: References: <20251113202320.2530531-1-mkhalfella@purestorage.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20251113202320.2530531-1-mkhalfella@purestorage.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251116_183052_200573_D0EF6FC7 X-CRM114-Status: GOOD ( 39.60 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On Thu, Nov 13, 2025 at 12:23:20PM -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. > > Convert set->tag_list_lock mutex to set->tag_list_rwsem rwsemaphore to > avoid the deadlock. Update blk_mq_[un]quiesce_tagset() to take the > semaphore for read since this is enough to guarantee no queues will be > added or removed. Update blk_mq_{add,del}_queue_tag_set() to take the > semaphore for write while updating set->tag_list and downgrade it to > read while freezing the queues. It should be safe to update set->flags > and hctx->flags while holding the semaphore for read since the queues > are already frozen. > > Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") > Signed-off-by: Mohamed Khalfella > --- > block/blk-mq-sysfs.c | 10 +++---- > block/blk-mq.c | 63 ++++++++++++++++++++++-------------------- > include/linux/blk-mq.h | 4 +-- > 3 files changed, 40 insertions(+), 37 deletions(-) > > diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c > index 58ec293373c6..f474781654fb 100644 > --- a/block/blk-mq-sysfs.c > +++ b/block/blk-mq-sysfs.c > @@ -230,13 +230,13 @@ int blk_mq_sysfs_register(struct gendisk *disk) > > kobject_uevent(q->mq_kobj, KOBJ_ADD); > > - mutex_lock(&q->tag_set->tag_list_lock); > + down_read(&q->tag_set->tag_list_rwsem); > queue_for_each_hw_ctx(q, hctx, i) { > ret = blk_mq_register_hctx(hctx); > if (ret) > goto out_unreg; > } > - mutex_unlock(&q->tag_set->tag_list_lock); > + up_read(&q->tag_set->tag_list_rwsem); > return 0; > > out_unreg: > @@ -244,7 +244,7 @@ int blk_mq_sysfs_register(struct gendisk *disk) > if (j < i) > blk_mq_unregister_hctx(hctx); > } > - mutex_unlock(&q->tag_set->tag_list_lock); > + up_read(&q->tag_set->tag_list_rwsem); > > kobject_uevent(q->mq_kobj, KOBJ_REMOVE); > kobject_del(q->mq_kobj); > @@ -257,10 +257,10 @@ void blk_mq_sysfs_unregister(struct gendisk *disk) > struct blk_mq_hw_ctx *hctx; > unsigned long i; > > - mutex_lock(&q->tag_set->tag_list_lock); > + down_read(&q->tag_set->tag_list_rwsem); > queue_for_each_hw_ctx(q, hctx, i) > blk_mq_unregister_hctx(hctx); > - mutex_unlock(&q->tag_set->tag_list_lock); > + up_read(&q->tag_set->tag_list_rwsem); > > kobject_uevent(q->mq_kobj, KOBJ_REMOVE); > kobject_del(q->mq_kobj); > diff --git a/block/blk-mq.c b/block/blk-mq.c > index d626d32f6e57..1770277fe453 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); > + down_read(&set->tag_list_rwsem); > list_for_each_entry(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); > + up_read(&set->tag_list_rwsem); > > 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); > + down_read(&set->tag_list_rwsem); > list_for_each_entry(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); > + up_read(&set->tag_list_rwsem); > } > EXPORT_SYMBOL_GPL(blk_mq_unquiesce_tagset); > > @@ -4280,7 +4280,7 @@ static void blk_mq_update_tag_set_shared(struct blk_mq_tag_set *set, > struct request_queue *q; > unsigned int memflags; > > - lockdep_assert_held(&set->tag_list_lock); > + lockdep_assert_held(&set->tag_list_rwsem); > > list_for_each_entry(q, &set->tag_list, tag_set_list) { > memflags = blk_mq_freeze_queue(q); > @@ -4293,37 +4293,40 @@ 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); > + down_write(&set->tag_list_rwsem); > list_del(&q->tag_set_list); > - if (list_is_singular(&set->tag_list)) { > - /* just transitioned to unshared */ > - set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED; > - /* update existing queue */ > - blk_mq_update_tag_set_shared(set, false); > + if (!list_is_singular(&set->tag_list)) { > + up_write(&set->tag_list_rwsem); > + goto out; > } > - mutex_unlock(&set->tag_list_lock); > + > + /* Transitioning to unshared. */ > + set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED; > + downgrade_write(&set->tag_list_rwsem); > + blk_mq_update_tag_set_shared(set, false); > + up_read(&set->tag_list_rwsem); > +out: > INIT_LIST_HEAD(&q->tag_set_list); > } > > static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set, > struct request_queue *q) > { > - mutex_lock(&set->tag_list_lock); > + down_write(&set->tag_list_rwsem); > + if (!list_is_singular(&set->tag_list)) { > + 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); > + up_write(&set->tag_list_rwsem); > + return; > + } It could be more readable to add documentation: /* * Three cases: * 1. Empty list (first queue): Add without shared flag * 2. Singular list (1→2 queues): Transition to shared * 3. Multiple queues (2+): Add with existing shared flag */ > > - /* > - * Check to see if we're transitioning to shared (from 1 to 2 queues). > - */ > - if (!list_empty(&set->tag_list) && > - !(set->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) { > - set->flags |= BLK_MQ_F_TAG_QUEUE_SHARED; > - /* update existing queue */ > - blk_mq_update_tag_set_shared(set, true); > - } > - if (set->flags & BLK_MQ_F_TAG_QUEUE_SHARED) > - queue_set_hctx_shared(q, true); > + /* Transitioning to shared. */ > + set->flags |= BLK_MQ_F_TAG_QUEUE_SHARED; > list_add_tail(&q->tag_set_list, &set->tag_list); > - > - mutex_unlock(&set->tag_list_lock); > + downgrade_write(&set->tag_list_rwsem); > + blk_mq_update_tag_set_shared(set, true); > + up_read(&set->tag_list_rwsem); > } Here you change to freeze two queues, and the in-tree code just freezes the 1st added queue. Probably it is nice to keep existing behavior to just freeze the 1st old queue. Also blk_mq_update_tag_set_shared() is only done on single queue, so the helper can be removed and just open-code queue_set_hctx_shared(), which can show the single queue point obviously and more readable. Otherwise, feel free to add: Reviewed-by: Ming Lei Thanks, Ming