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 0B5CD35959 for ; Tue, 9 Dec 2025 04:04:38 +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=1765253080; cv=none; b=SIeKSZ2lSReFLWEnk3cJFmCUWMVL6W/UcooEEiU/jF2qQoZjkRrrjMkT34jFV6MHKZGX29lQILYiRBNJERQDusvlaSPPjV0AGvSlstcCo3Lyi90ryVoVyW3ETYBD5n20ciiGlU6yXClUbUVpqYcfn+nuoklh/lME0vLLHL+f8hI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765253080; c=relaxed/simple; bh=ru9bq3/XjjaBzixorAgUhmDiiYoWiQIInMUv/mKBOUI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tjkucC62vYiXCmUdAlmlNHRkJ505ky9Bf7e+95B9Flvf6QWJJf7GyawWRLgkOwH9Dq5vPJJQryc6uejFnLAJU0J3jm1pUpAaj+eBh9NeRXxIFQv5U6EfaFOjd8Km/Hkq9N05GAZB7WxU+nMmAePdZYUsWAtk2DkglfEuDaifxNY= 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=SmphT7IE; 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="SmphT7IE" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1765253078; 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: in-reply-to:in-reply-to:references:references; bh=0nbVlvCnhTFfBFfYizweUOs3ZcBOdqLEtYuSExW8Bpk=; b=SmphT7IETIALuUnoiBNqGBt/o3phW49oEp+J3HaLI7lBDCJWkIy9zvHOKwzb2z3tUDx5Rj Pq1Gtg5PFr8WJQtId8bddxVlCZkRd2bNfuI9mFy2NcscHZJd6HpflbHQTyHAP8tNZNnX42 Hp/uAm7l8sJD4nF+6WIHLtMh9txhZA4= 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-605-Un38cTO2P-6ZqPGHjICTLA-1; Mon, 08 Dec 2025 23:04:34 -0500 X-MC-Unique: Un38cTO2P-6ZqPGHjICTLA-1 X-Mimecast-MFC-AGG-ID: Un38cTO2P-6ZqPGHjICTLA_1765253072 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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 90FE11800637; Tue, 9 Dec 2025 04:04:31 +0000 (UTC) Received: from fedora (unknown [10.72.116.98]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5ACE3180044F; Tue, 9 Dec 2025 04:04:22 +0000 (UTC) Date: Tue, 9 Dec 2025 12:04:18 +0800 From: Ming Lei To: Mohamed Khalfella Cc: Chaitanya Kulkarni , Christoph Hellwig , Jens Axboe , Keith Busch , Sagi Grimberg , Casey Chen , Yuanyuan Zhong , Hannes Reinecke , Waiman Long , Hillf Danton , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 1/1] block: Use RCU in blk_mq_[un]quiesce_tagset() instead of set->tag_list_lock Message-ID: References: <20251205211738.1872244-1-mkhalfella@purestorage.com> <20251205211738.1872244-2-mkhalfella@purestorage.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: <20251205211738.1872244-2-mkhalfella@purestorage.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On Fri, Dec 05, 2025 at 01:17:02PM -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+0x47c/0xbb0 > ? timerqueue_add+0x66/0xb0 > schedule+0x1c/0xa0 > schedule_preempt_disabled+0xa/0x10 > __mutex_lock.constprop.0+0x271/0x600 > blk_mq_quiesce_tagset+0x25/0xc0 > nvme_dev_disable+0x9c/0x250 > nvme_timeout+0x1fc/0x520 > blk_mq_handle_expired+0x5c/0x90 > bt_iter+0x7e/0x90 > blk_mq_queue_tag_busy_iter+0x27e/0x550 > ? __blk_mq_complete_request_remote+0x10/0x10 > ? __blk_mq_complete_request_remote+0x10/0x10 > ? __call_rcu_common.constprop.0+0x1c0/0x210 > blk_mq_timeout_work+0x12d/0x170 > process_one_work+0x12e/0x2d0 > worker_thread+0x288/0x3a0 > ? rescuer_thread+0x480/0x480 > kthread+0xb8/0xe0 > ? kthread_park+0x80/0x80 > ret_from_fork+0x2d/0x50 > ? kthread_park+0x80/0x80 > ret_from_fork_asm+0x11/0x20 > > __schedule+0x47c/0xbb0 > ? xas_find+0x161/0x1a0 > schedule+0x1c/0xa0 > blk_mq_freeze_queue_wait+0x3d/0x70 > ? destroy_sched_domains_rcu+0x30/0x30 > blk_mq_update_tag_set_shared+0x44/0x80 > blk_mq_exit_queue+0x141/0x150 > del_gendisk+0x25a/0x2d0 > nvme_ns_remove+0xc9/0x170 > nvme_remove_namespaces+0xc7/0x100 > nvme_remove+0x62/0x150 > pci_device_remove+0x23/0x60 > device_release_driver_internal+0x159/0x200 > unbind_store+0x99/0xa0 > kernfs_fop_write_iter+0x112/0x1e0 > vfs_write+0x2b1/0x3d0 > ksys_write+0x4e/0xb0 > do_syscall_64+0x5b/0x160 > entry_SYSCALL_64_after_hwframe+0x4b/0x53 > > 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 tries 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]quiescing 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, update blk_mq_{add,del}_queue_tag_set() to use RCU > safe list operations. Also, delete INIT_LIST_HEAD(&q->tag_set_list) > in blk_mq_del_queue_tag_set() because we can not re-initialize it while > the list is being traversed under RCU. The deleted queue will not be > added/deleted to/from a tagset and it will be freed in blk_free_queue() > after the end of RCU grace period. > > Signed-off-by: Mohamed Khalfella > Fixes: 98d81f0df70c ("nvme: use blk_mq_[un]quiesce_tagset") Looks fine: Reviewed-by: Ming Lei Thanks, Ming