linux-kernel - Re: [PATCH] nbd: fix false lockdep deadlock warning

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Message-ID: <99b4afce-de05-ddcb-2634-b19214cf4534@huaweicloud.com>
Date: Wed, 2 Jul 2025 09:12:09 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Nilay Shroff <nilay@...ux.ibm.com>, Yu Kuai <yukuai1@...weicloud.com>,
 Ming Lei <ming.lei@...hat.com>
Cc: josef@...icpanda.com, axboe@...nel.dk, hch@...radead.org, hare@...e.de,
 linux-block@...r.kernel.org, nbd@...er.debian.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] nbd: fix false lockdep deadlock warning

Hi,

在 2025/07/01 21:28, Nilay Shroff 写道:
> 
> 
> On 6/28/25 6:18 AM, Yu Kuai wrote:
>> Hi,
>>
>> 在 2025/06/27 19:04, Ming Lei 写道:
>>> I guess the patch in the following link may be simper, both two take
>>> similar approach:
>>>
>>> https://lore.kernel.org/linux-block/aFjbavzLAFO0Q7n1@fedora/
>>
>> I this the above approach has concurrent problems if nbd_start_device
>> concurrent with nbd_start_device:
>>
>> t1:
>> nbd_start_device
>> lock
>> num_connections = 1
>> unlock
>>      t2:
>>      nbd_add_socket
>>      lock
>>      config->num_connections++
>>      unlock
>>          t3:
>>          nbd_start_device
>>          lock
>>          num_connections = 2
>>          unlock
>>          blk_mq_update_nr_hw_queues
>>
>> blk_mq_update_nr_hw_queues
>> //nr_hw_queues updated to 1 before failure
>> return -EINVAL
>>
> 
> In the above case, yes I see that t1 would return -EINVAL (as
> config->num_connections doesn't match with num_connections)
> but then t3 would succeed to update nr_hw_queue (as both
> config->num_connections and num_connections set to 2 this
> time). Isn't it? If yes, then the above patch (from Ming)
> seems good.

Emm, I'm confused, If you agree with the concurrent process, then
t3 update nr_hw_queues to 2 first and return sucess, later t1 update
nr_hw_queues back to 1 and return failure.

Thanks,
Kuai

> 
> Thanks,
> --Nilay
> .
>