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Wed, 9 Apr 2025 09:12:07 +0000 (GMT) Message-ID: Date: Wed, 9 Apr 2025 14:42:06 +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-ORIG-GUID: 3C7WqC12Cb2ThLydGvEEFETpiahC8K0e X-Proofpoint-GUID: 3C7WqC12Cb2ThLydGvEEFETpiahC8K0e 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=2025-04-09_03,2025-04-08_04,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 spamscore=0 malwarescore=0 suspectscore=0 impostorscore=0 priorityscore=1501 adultscore=0 bulkscore=0 mlxlogscore=999 mlxscore=0 phishscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2502280000 definitions=main-2504090046 On 4/8/25 7:20 PM, Ming Lei wrote: > On Tue, Apr 08, 2025 at 06:55:26PM +0530, Nilay Shroff wrote: >> >> >> 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. > > How can you make sure all NSs are covered in this way? RCU/SRCU can't > provide such kind of guarantee. > Why is that so? In fact, nvme_wait_freeze also iterates through the same ctrl->namespaces to freeze the queue. >> >>> 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? > > Then why do you think per-set lock can solve the lockdep splat? > > __blk_mq_update_nr_hw_queues is the only chance for tagset wide queues > involved wrt. switching elevator. If elevator switching is not allowed > when __blk_mq_update_nr_hw_queues() is started, why do we need per-set > lock? > Yes if elevator switch is not allowed then we probably don't need per-set lock. However my question was if we were to not allow elevator switch while __blk_mq_update_nr_hw_queues is running then how would we implement it? Do we need to synchronize with ->tag_list_lock? Or in another words, elv_iosched_store would now depends on ->tag_list_lock ? On another note, if we choose to make ->elevator_lock per-set then our locking sequence in blk_mq_update_nr_hw_queues() would be, blk_mq_update_nr_hw_queues -> tag_list_lock -> elevator_lock -> freeze_lock elv_iosched_store -> elevator_lock -> freeze_lock So now ->freeze_lock should not depend on ->elevator_lock and that shall help avoid few of the recent lockdep splats reported with fs_reclaim. What do you think? Thanks, --Nilay