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

[Yu Kuai]

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
> .