Re: [PATCH] nbd: fix false lockdep deadlock warning

[Nilay Shroff <nilay@linux.ibm.com>]

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. 

Thanks,
--Nilay