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

From: Jens Axboe <[email-masked]>
Date: Wed, 3 Nov 2021 09:10:17 -0600

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 <[email-masked]> wrote:
>>>>
>>>> On 11/2/21 9:54 PM, Jens Axboe wrote:
>>>>> On Nov 2, 2021, at 9:52 PM, Ming Lei <[email-masked]> 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,
 		};
 
 		if (unlikely(!blk_try_enter_queue(q, false) &&

-- 
Jens Axboe