Re: [bug report] WARNING: CPU: 1 PID: 1386 at block/blk-mq-sched.c:432 blk_mq_sched_insert_request+0x54/0x178

Jens Axboe
2021-11-03T15:49:23Z

On 11/3/21 9:41 AM, Jens Axboe wrote:
> On 11/3/21 9:16 AM, Ming Lei wrote:
>> On Wed, Nov 03, 2021 at 09:10:17AM -0600, Jens Axboe wrote:
>>> On 11/3/21 9:03 AM, Jens Axboe wrote:
>>>> On 11/3/21 8:57 AM, Ming Lei wrote:
>>>>> On Wed, Nov 03, 2021 at 09:59:02PM +0800, Yi Zhang wrote:
>>>>>> On Wed, Nov 3, 2021 at 7:59 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>>
>>>>>>> On 11/2/21 9:54 PM, Jens Axboe wrote:
>>>>>>>> On Nov 2, 2021, at 9:52 PM, Ming Lei <ming.lei@redhat.com> wrote:
>>>>>>>>>
>>>>>>>>> ﻿On Tue, Nov 02, 2021 at 09:21:10PM -0600, Jens Axboe wrote:
>>>>>>>>>>> On 11/2/21 8:21 PM, Yi Zhang wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> Can either one of you try with this patch? Won't fix anything, but it'll
>>>>>>>>>>>>> hopefully shine a bit of light on the issue.
>>>>>>>>>>>>>
>>>>>>>>>>> Hi Jens
>>>>>>>>>>>
>>>>>>>>>>> Here is the full log:
>>>>>>>>>>
>>>>>>>>>> Thanks! I think I see what it could be - can you try this one as well,
>>>>>>>>>> would like to confirm that the condition I think is triggering is what
>>>>>>>>>> is triggering.
>>>>>>>>>>
>>>>>>>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>>>>>>>> index 07eb1412760b..81dede885231 100644
>>>>>>>>>> --- a/block/blk-mq.c
>>>>>>>>>> +++ b/block/blk-mq.c
>>>>>>>>>> @@ -2515,6 +2515,8 @@ void blk_mq_submit_bio(struct bio *bio)
>>>>>>>>>> if (plug && plug->cached_rq) {
>>>>>>>>>> rq = rq_list_pop(&plug->cached_rq);
>>>>>>>>>> INIT_LIST_HEAD(&rq->queuelist);
>>>>>>>>>> + WARN_ON_ONCE(q->elevator && !(rq->rq_flags & RQF_ELV));
>>>>>>>>>> + WARN_ON_ONCE(!q->elevator && (rq->rq_flags & RQF_ELV));
>>>>>>>>>> } else {
>>>>>>>>>> struct blk_mq_alloc_data data = {
>>>>>>>>>> .q = q,
>>>>>>>>>> @@ -2535,6 +2537,8 @@ void blk_mq_submit_bio(struct bio *bio)
>>>>>>>>>> bio_wouldblock_error(bio);
>>>>>>>>>> goto queue_exit;
>>>>>>>>>> }
>>>>>>>>>> + WARN_ON_ONCE(q->elevator && !(rq->rq_flags & RQF_ELV));
>>>>>>>>>> + WARN_ON_ONCE(!q->elevator && (rq->rq_flags & RQF_ELV));
>>>>>>>>>
>>>>>>>>> Hello Jens,
>>>>>>>>>
>>>>>>>>> I guess the issue could be the following code run without grabbing
>>>>>>>>> ->q_usage_counter from blk_mq_alloc_request() and blk_mq_alloc_request_hctx().
>>>>>>>>>
>>>>>>>>> .rq_flags = q->elevator ? RQF_ELV : 0,
>>>>>>>>>
>>>>>>>>> then elevator is switched to real one from none, and check on q->elevator
>>>>>>>>> becomes not consistent.
>>>>>>>>
>>>>>>>> Indeed, that’s where I was going with this. I have a patch, testing it
>>>>>>>> locally but it’s getting late. Will send it out tomorrow. The nice
>>>>>>>> benefit is that it allows dropping the weird ref get on plug flush,
>>>>>>>> and batches getting the refs as well.
>>>>>>>
>>>>>>> Yi/Steffen, can you try pulling this into your test kernel:
>>>>>>>
>>>>>>> git://git.kernel.dk/linux-block for-next
>>>>>>>
>>>>>>> and see if it fixes the issue for you. Thanks!
>>>>>>
>>>>>> It still can be reproduced with the latest linux-block/for-next, here is the log
>>>>>>
>>>>>> fab2914e46eb (HEAD, new/for-next) Merge branch 'for-5.16/drivers' into for-next
>>>>>
>>>>> Hi Yi,
>>>>>
>>>>> Please try the following change:
>>>>>
>>>>>
>>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>>> index e1e64964a31b..eb634a9c61ff 100644
>>>>> --- a/block/blk-mq.c
>>>>> +++ b/block/blk-mq.c
>>>>> @@ -494,7 +494,6 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
>>>>> .q = q,
>>>>> .flags = flags,
>>>>> .cmd_flags = op,
>>>>> - .rq_flags = q->elevator ? RQF_ELV : 0,
>>>>> .nr_tags = 1,
>>>>> };
>>>>> struct request *rq;
>>>>> @@ -504,6 +503,7 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
>>>>> if (ret)
>>>>> return ERR_PTR(ret);
>>>>>
>>>>> + data.rq_flags = q->elevator ? RQF_ELV : 0,
>>>>> rq = __blk_mq_alloc_requests(&data);
>>>>> if (!rq)
>>>>> goto out_queue_exit;
>>>>> @@ -524,7 +524,6 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>>>> .q = q,
>>>>> .flags = flags,
>>>>> .cmd_flags = op,
>>>>> - .rq_flags = q->elevator ? RQF_ELV : 0,
>>>>> .nr_tags = 1,
>>>>> };
>>>>> u64 alloc_time_ns = 0;
>>>>> @@ -551,6 +550,7 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>>>> ret = blk_queue_enter(q, flags);
>>>>> if (ret)
>>>>> return ERR_PTR(ret);
>>>>> + data.rq_flags = q->elevator ? RQF_ELV : 0,
>>>>
>>>> Don't think that will compile, but I guess the point is that we can't do
>>>> this assignment before queue enter, in case we're in the midst of
>>>> switching schedulers. Which is indeed a valid concern.
>>>
>>> Something like the below. Maybe? On top of the for-next that was already
>>> pulled in.
>>>
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index b01e05e02277..121f1898d529 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -433,9 +433,11 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
>>> if (data->cmd_flags & REQ_NOWAIT)
>>> data->flags |= BLK_MQ_REQ_NOWAIT;
>>>
>>> - if (data->rq_flags & RQF_ELV) {
>>> + if (q->elevator) {
>>> struct elevator_queue *e = q->elevator;
>>>
>>> + data->rq_flags |= RQF_ELV;
>>> +
>>> /*
>>> * Flush/passthrough requests are special and go directly to the
>>> * dispatch list. Don't include reserved tags in the
>>> @@ -494,7 +496,6 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
>>> .q = q,
>>> .flags = flags,
>>> .cmd_flags = op,
>>> - .rq_flags = q->elevator ? RQF_ELV : 0,
>>> .nr_tags = 1,
>>> };
>>> struct request *rq;
>>> @@ -524,7 +525,6 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>> .q = q,
>>> .flags = flags,
>>> .cmd_flags = op,
>>> - .rq_flags = q->elevator ? RQF_ELV : 0,
>>> .nr_tags = 1,
>>> };
>>> u64 alloc_time_ns = 0;
>>> @@ -565,6 +565,8 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>>>
>>> if (!q->elevator)
>>> blk_mq_tag_busy(data.hctx);
>>> + else
>>> + data.rq_flags |= RQF_ELV;
>>>
>>> ret = -EWOULDBLOCK;
>>> tag = blk_mq_get_tag(&data);
>>> @@ -2560,7 +2562,6 @@ void blk_mq_submit_bio(struct bio *bio)
>>> .q = q,
>>> .nr_tags = 1,
>>> .cmd_flags = bio->bi_opf,
>>> - .rq_flags = q->elevator ? RQF_ELV : 0,
>>> };
>>
>> The above patch looks fine.
>>
>> BTW, 9ede85cb670c ("block: move queue enter logic into
>> blk_mq_submit_bio()") moves the queue enter into blk_mq_submit_bio(),
>> which seems dangerous, especially blk_mq_sched_bio_merge() needs
>> hctx/ctx which requires q_usage_counter to be grabbed.
>
> I think the best solution is to enter just for that as well, and just
> retain that enter state. I'll update the patch, there's some real fixes
> in there too for the batched alloc. Will post them later today.

Is it needed, though? As far as I can tell, it's only needed
persistently for having the IO inflight, otherwise if the premise is
that the queue can just go away, we're in trouble before that too. And I
don't think that's the case.

--
Jens Axboe