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Tue, 8 Apr 2025 13:25:27 +0000 (GMT) Message-ID: Date: Tue, 8 Apr 2025 18:55:26 +0530 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] block: don't grab elevator lock during queue initialization To: Ming Lei Cc: Christoph Hellwig , Jens Axboe , linux-block@vger.kernel.org, syzbot+4c7e0f9b94ad65811efb@syzkaller.appspotmail.com References: <20250403105402.1334206-1-ming.lei@redhat.com> <20250404091037.GB12163@lst.de> <92feba7e-84fc-4668-92c3-aba4e8320559@linux.ibm.com> Content-Language: en-US From: Nilay Shroff In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: OGDeAN6SehBs2AZYRPwwyz-OEB6HX93T X-Proofpoint-ORIG-GUID: OGDeAN6SehBs2AZYRPwwyz-OEB6HX93T X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1095,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-04-08_05,2025-04-08_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 spamscore=0 adultscore=0 suspectscore=0 mlxlogscore=999 bulkscore=0 clxscore=1015 malwarescore=0 mlxscore=0 priorityscore=1501 phishscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2502280000 definitions=main-2504080092 On 4/8/25 1:08 PM, Ming Lei wrote: > On Mon, Apr 07, 2025 at 01:59:48PM +0530, Nilay Shroff wrote: >> >> >> On 4/7/25 8:39 AM, Ming Lei wrote: >>> On Sat, Apr 05, 2025 at 07:44:19PM +0530, Nilay Shroff wrote: >>>> >>>> >>>> On 4/4/25 2:40 PM, Christoph Hellwig wrote: >>>>> On Thu, Apr 03, 2025 at 06:54:02PM +0800, Ming Lei wrote: >>>>>> Fixes the following lockdep warning: >>>>> >>>>> Please spell the actual dependency out here, links are not permanent >>>>> and also not readable for any offline reading of the commit logs. >>>>> >>>>>> +static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, >>>>>> + struct request_queue *q, bool lock) >>>>>> +{ >>>>>> + if (lock) { >>>>> >>>>> bool lock(ed) arguments are an anti-pattern, and regularly get Linus >>>>> screaming at you (in this case even for the right reason :)) >>>>> >>>>>> + /* protect against switching io scheduler */ >>>>>> + mutex_lock(&q->elevator_lock); >>>>>> + __blk_mq_realloc_hw_ctxs(set, q); >>>>>> + mutex_unlock(&q->elevator_lock); >>>>>> + } else { >>>>>> + __blk_mq_realloc_hw_ctxs(set, q); >>>>>> + } >>>>> >>>>> I think the problem here is again that because of all the other >>>>> dependencies elevator_lock really needs to be per-set instead of >>>>> per-queue which will allows us to have much saner locking hierarchies. >>>>> >>>> I believe you meant here q->tag_set->elevator_lock? >>> >>> I don't know what locks you are planning to invent. >>> >>> For set->tag_list_lock, it has been very fragile: >>> >>> blk_mq_update_nr_hw_queues >>> set->tag_list_lock >>> freeze_queue >>> >>> If IO failure happens when waiting in above freeze_queue(), the nvme error >>> handling can't provide forward progress any more, because the error >>> handling code path requires set->tag_list_lock. >> >> I think you're referring here nvme_quiesce_io_queues and nvme_unquiesce_io_queues > > Yes. > >> which is called in nvme error handling path. If yes then I believe this function >> could be easily modified so that it doesn't require ->tag_list_lock. > > Not sure it is easily, ->tag_list_lock is exactly for protecting the list of "set->tag_list". > Please see this, here nvme_quiesce_io_queues doen't require ->tag_list_lock: diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c index 777db89fdaa7..002d2fd20e0c 100644 --- a/drivers/nvme/host/core.c +++ b/drivers/nvme/host/core.c @@ -5010,10 +5010,19 @@ void nvme_quiesce_io_queues(struct nvme_ctrl *ctrl) { if (!ctrl->tagset) return; - if (!test_and_set_bit(NVME_CTRL_STOPPED, &ctrl->flags)) - blk_mq_quiesce_tagset(ctrl->tagset); - else - blk_mq_wait_quiesce_done(ctrl->tagset); + if (!test_and_set_bit(NVME_CTRL_STOPPED, &ctrl->flags)) { + struct nvme_ns *ns; + int srcu_idx; + + srcu_idx = srcu_read_lock(&ctrl->srcu); + list_for_each_entry_srcu(ns, &ctrl->namespaces, list, + srcu_read_lock_held(&ctrl->srcu)) { + if (!blk_queue_skip_tagset_quiesce(ns->queue)) + blk_mq_quiesce_queue_nowait(ns->queue); + } + srcu_read_unlock(&ctrl->srcu, srcu_idx); + } + blk_mq_wait_quiesce_done(ctrl->tagset); } EXPORT_SYMBOL_GPL(nvme_quiesce_io_queues); Here we iterate through ctrl->namespaces instead of relying on tag_list and so we don't need to acquire ->tag_list_lock. > And the same list is iterated in blk_mq_update_nr_hw_queues() too. > >> >>> >>> So all queues should be frozen first before calling blk_mq_update_nr_hw_queues, >>> fortunately that is what nvme is doing. >>> >>> >>>> If yes then it means that we should be able to grab ->elevator_lock >>>> before freezing the queue in __blk_mq_update_nr_hw_queues and so locking >>>> order should be in each code path, >>>> >>>> __blk_mq_update_nr_hw_queues >>>> ->elevator_lock >>>> ->freeze_lock >>> >>> Now tagset->elevator_lock depends on set->tag_list_lock, and this way >>> just make things worse. Why can't we disable elevator switch during >>> updating nr_hw_queues? >>> >> I couldn't quite understand this. As we already first disable the elevator >> before updating sw to hw queue mapping in __blk_mq_update_nr_hw_queues(). >> Once mapping is successful we switch back the elevator. > > Yes, but user still may switch elevator from none to others during the > period, right? > Yes correct, that's possible. So your suggestion was to disable elevator update while we're running __blk_mq_update_nr_hw_queues? And that way user couldn't update elevator through sysfs (elv_iosched_store) while we update nr_hw_queues? If this is true then still how could it help solve lockdep splat? Thanks, --Nilay