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

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

On 7/2/25 8:02 AM, Ming Lei wrote:
> On Wed, Jul 02, 2025 at 09:12:09AM +0800, Yu Kuai wrote:
>> 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.
> 
> It should be easy to avoid failure by simple retrying.
> 
Yeah I think retry should be a safe bet here. 

On another note, synchronizing nbd_start_device and nbd_add_socket
using nbd->task_setup looks more complex and rather we may use 
nbd->pid to synchronize both. We need to move setting of nbd->pid
before we invoke blk_mq_update_nr_hw_queues in nbd_start_device.
Then in nbd_add_socket we can evaluate nbd->pid and if it's 
non-NULL then we could assume that either nr_hw_queues update is in 
progress or device has been setup and so return -EBUSY. I think
anyways updating number of connections once device is configured
would not be possible, so once nbd_start_device is initiated, we
shall prevent user adding more connections. If we follow this
approach then IMO we don't need to add retry discussed above.

Thanks,
--Nilay