Re: [REPORT] Possible circular locking dependency on 6.18-rc2 in blkg_conf_open_bdev_frozen+0x80/0xa0

[Ming Lei <ming.lei@redhat.com>]

On Tue, Oct 28, 2025 at 06:36:20PM +0530, Nilay Shroff wrote:
> 
> 
> On 10/28/25 2:00 AM, Bart Van Assche wrote:
> > On 10/23/25 9:54 PM, Nilay Shroff wrote:
> >> IMO, we need to make lockdep learn about this differences by assigning separate
> >> lockdep key/class for each queue's q->debugfs_mutex to avoid this false positive.
> >> As this is another report with the same false-positive lockdep splat, I think we
> >> should address this.
> >>
> >> Any other thoughts or suggestions from others on the list?
> > 
> > Please take a look at lockdep_register_key() and
> > lockdep_unregister_key(). I introduced these functions six years ago to
> > suppress false positive lockdep complaints like this one.
> > 
> Thanks Bart! I'll send out patch with the above proposed fix.

IMO, that may not be a smart approach, here the following dependency should
be cut:

#4 (&q->q_usage_counter(io)#2){++++}-{0:0}:

...

#1 (&q->debugfs_mutex){+.+.}-{4:4}:
#0 (&q->rq_qos_mutex){+.+.}-{4:4}:

Why is there the dependency between `#1 (&q->debugfs_mutex)` and `#0 (&q->rq_qos_mutex)`?

I remember that Yu Kuai is working on remove it:

https://lore.kernel.org/linux-block/20251014022149.947800-1-yukuai3@huawei.com/


Thanks,
Ming