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

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Message-ID: <aGSaVhiH2DeTvtdr@fedora>
Date: Wed, 2 Jul 2025 10:32:54 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: Nilay Shroff <nilay@...ux.ibm.com>, 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

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.


Thanks,
Ming