From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 DBEF626CE34 for ; Tue, 18 Nov 2025 03:09:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763435353; cv=none; b=AoyiYJg5gwOIhpRxUDsF4eD7ZvbRRdW8pizoUT1+YlqeSK05JazxOgo3gi1Tsc2wqiKggqKeGB0jprlGuU3pLz3X8+udMlr/ZDTWbZ7/mFOHthOecRoAFFaGOT4Z/cfvbi2AiKPkHHlsZOF/Nmrsk7e8z1cWygq0/c00hIRCUn0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763435353; c=relaxed/simple; bh=dxAeQcTDEzK6aFYZ+m53cImLOBYDKqob2sy9PRPzVpU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=PVWbNXeXxeoiBOkeLQbA20GX06XQnQqzUR3p495cOa7RGnwLsBC6LsIDHSvTaMX8vNu/JCPpBZ7R1YhHcb6zW23523CQBRqEsN0KHKYwOgLJHWWJ8s9ALki3BnPycsl2i8tQBFDRUMKVphtU1sEH2K6wlcfLwESOi8uJgiG/uOs= 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=QORhdUal; arc=none smtp.client-ip=170.10.133.124 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="QORhdUal" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1763435350; 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=u18Y5XK4NGsQA2qOH/XAy6l1ox3/qT2LljFhlIdk1kU=; b=QORhdUalzypsgZSz60cgeTFX4Cfmv7jRVSnc5Hd7umQMTL8X7dFd0swhZKbZQPwkOnBHRq D7krcR6FNP/oRS+5F5RXQtpe7FvusJN7z4HNecEmQxQnrJV5YKYsbEXnSOcIvMEBMFd7f7 +GO5h60uIPJjsKmB8fEPbLZG/nWRUZ8= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-96-u8dhLOsXOvq2TOgMLMPw7A-1; Mon, 17 Nov 2025 22:09:05 -0500 X-MC-Unique: u8dhLOsXOvq2TOgMLMPw7A-1 X-Mimecast-MFC-AGG-ID: u8dhLOsXOvq2TOgMLMPw7A_1763435343 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-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0928719560A3; Tue, 18 Nov 2025 03:09:02 +0000 (UTC) Received: from fedora (unknown [10.72.116.204]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id F1F611800451; Tue, 18 Nov 2025 03:08:56 +0000 (UTC) Date: Tue, 18 Nov 2025 11:08:51 +0800 From: Ming Lei To: Waiman Long Cc: Hillf Danton , Mohamed Khalfella , Jens Axboe , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] nvme: Convert tag_list mutex to rwsemaphore to avoid deadlock Message-ID: References: <20251117202414.4071380-1-mkhalfella@purestorage.com> <20251118013442.9414-1-hdanton@sina.com> <5db3bb06-0bf2-4ba3-b765-c217acda1b0c@redhat.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: <5db3bb06-0bf2-4ba3-b765-c217acda1b0c@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On Mon, Nov 17, 2025 at 09:24:21PM -0500, Waiman Long wrote: > On 11/17/25 8:34 PM, Hillf Danton wrote: > > On Mon, 17 Nov 2025 12:23:53 -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. > > > > > > 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 | 95 +++++++++++++++++++++++------------------- > > > include/linux/blk-mq.h | 4 +- > > > 3 files changed, 58 insertions(+), 51 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..9211d32ce820 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); > > > @@ -4274,56 +4274,63 @@ static void queue_set_hctx_shared(struct request_queue *q, bool shared) > > > } > > > } > > > -static void blk_mq_update_tag_set_shared(struct blk_mq_tag_set *set, > > > - bool shared) > > > -{ > > > - struct request_queue *q; > > > - unsigned int memflags; > > > - > > > - lockdep_assert_held(&set->tag_list_lock); > > > - > > > - list_for_each_entry(q, &set->tag_list, tag_set_list) { > > > - memflags = blk_mq_freeze_queue(q); > > > - queue_set_hctx_shared(q, shared); > > > - blk_mq_unfreeze_queue(q, memflags); > > > - } > > > -} > > > - > > > static void blk_mq_del_queue_tag_set(struct request_queue *q) > > > { > > > struct blk_mq_tag_set *set = q->tag_set; > > > + struct request_queue *firstq; > > > + unsigned int memflags; > > > - 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 the remaining firstq to unshared. > > > + * Also, downgrade the semaphore to avoid deadlock > > > + * with blk_mq_quiesce_tagset() while waiting for > > > + * firstq to be frozen. > > > + */ > > > + set->flags &= ~BLK_MQ_F_TAG_QUEUE_SHARED; > > > + downgrade_write(&set->tag_list_rwsem); > > If the first lock waiter is for write, it could ruin your downgrade trick. If the 1st waiter is for WEITE, rwsem_mark_wake() simply returns and grants read lock to this caller, meantime wakes up nothing. That is exactly what this use case expects, so can you explain in detail why `it could ruin your downgrade trick`? > > That is true. The downgrade will wake up all the waiting readers at the > front of the wait queue, but if there is one or more writers in the mix. The > wakeup will stop when the first writer is hit and all the readers after that > will not be woken up. So waiters for WRITE won't be waken up by downgrade_write() if I understand correctly, and rwsem_downgrade_wake() documents this behavior too. > > We can theoretically provide a downgrade variant that wakes up all the > readers if it is a useful feature. The following up_read() in this code block will wake up the waiter for WRITE, which finally wakes up other waiters for READ, then I am confused what is the problem with this usage? Thanks, Ming