linux-kernel - Re: [syzbot] [nbd?] possible deadlock in nbd_queue

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Message-ID: <20250708012450.2858-1-hdanton@sina.com>
Date: Tue,  8 Jul 2025 09:24:49 +0800
From: Hillf Danton <hdanton@...a.com>
To: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc: Bart Van Assche <bvanassche@....org>,
	axboe@...nel.dk,
	josef@...icpanda.com,
	linux-block@...r.kernel.org,
	syzbot <syzbot+3dbc6142c85cc77eaf04@...kaller.appspotmail.com>,
	Ming Lei <ming.lei@...hat.com>,
	linux-kernel@...r.kernel.org,
	nbd@...er.debian.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [nbd?] possible deadlock in nbd_queue_rq

On Tue, 8 Jul 2025 09:52:18 +0900 Tetsuo Handa wrote:
> On 2025/07/08 9:18, Hillf Danton wrote:
> > On Mon, 7 Jul 2025 10:39:44 -0700 Bart Van Assche wrote:
> >> On 7/6/25 5:59 PM, Hillf Danton wrote:
> >>> and given the second one, the report is false positive.
> >>
> >> Whether or not this report is a false positive, the root cause should be
> >> fixed because lockdep disables itself after the first circular locking
> >> complaint. From print_usage_bug() in kernel/locking/lockdep.c:
> >>
> >> 	if (!debug_locks_off() || debug_locks_silent)
> >> 		return;
> >>
> > The root cause could be walked around for example by trying not to init
> > nbd more than once.
> 
> How did you come to think so?
> 
Based on that nbd_init appears twice in the lock chain syzbot reported.

> nbd_init() is already called only once because of module_init(nbd_init).
> 
Ok Bart is misguiding.