Re: [PATCH for-6.18/block 08/10] blk-mq: fix potential deadlock while nr_requests grown

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/09/09 14:39, Nilay Shroff 写道:
> 
> 
> On 9/8/25 11:45 AM, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> Allocate and free sched_tags while queue is freezed can deadlock[1],
>> this is a long term problem, hence allocate memory before freezing
>> queue and free memory after queue is unfreezed.
>>
>> [1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/
>> Fixes: e3a2b3f931f5 ("blk-mq: allow changing of queue depth through sysfs")
>>
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> [...]
> [...]
> 
>> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
>> index 7ea15bf68b4b..a0a7ebad378f 100644
>> --- a/block/blk-sysfs.c
>> +++ b/block/blk-sysfs.c
>> @@ -64,11 +64,12 @@ static ssize_t queue_requests_show(struct gendisk *disk, char *page)
>>   static ssize_t
>>   queue_requests_store(struct gendisk *disk, const char *page, size_t count)
>>   {
>> -	unsigned long nr;
>> -	int ret, err;
>> -	unsigned int memflags;
>>   	struct request_queue *q = disk->queue;
>>   	struct blk_mq_tag_set *set = q->tag_set;
>> +	struct elevator_tags *et = NULL;
>> +	unsigned int memflags;
>> +	unsigned long nr;
>> +	int ret;
>>   
>>   	ret = queue_var_store(&nr, page, count);
>>   	if (ret < 0)
>> @@ -90,16 +91,24 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
>>   		goto unlock;
>>   	}
>>   
>> +	if (q->elevator && nr > q->elevator->et->nr_requests) {
>> +		/* allocate memory before freezing queue to prevent deadlock */
>> +		et = blk_mq_alloc_sched_tags(set, q->nr_hw_queues, nr);
>> +		if (!et) {
>> +			ret = -ENOMEM;
>> +			goto unlock;
>> +		}
>> +	}
>> +
> I think we should add a comment above explaining why is it safe
> to access q->elevator without holding ->elevator_lock.
> 

I already access q->elevator to check input nr from patch 4, and that's
why I add comments to explain switching elevator is serialized, is this
enough?

Thanks,
Kuai

> Thanks,
> --Nilay
> .
>