Re: [dm-devel] [PATCH 4/8] block: support delayed holder registration

[Guenter Roeck <linux@roeck-us.net>]

On 8/15/21 12:07 AM, Christoph Hellwig wrote:
> On Sat, Aug 14, 2021 at 02:13:09PM -0700, Guenter Roeck wrote:
>> On Wed, Aug 04, 2021 at 11:41:43AM +0200, Christoph Hellwig wrote:
>>> device mapper needs to register holders before it is ready to do I/O.
>>> Currently it does so by registering the disk early, which can leave
>>> the disk and queue in a weird half state where the queue is registered
>>> with the disk, except for sysfs and the elevator.  And this state has
>>> been a bit promlematic before, and will get more so when sorting out
>>> the responsibilities between the queue and the disk.
>>>
>>> Support registering holders on an initialized but not registered disk
>>> instead by delaying the sysfs registration until the disk is registered.
>>>
>>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>>> Reviewed-by: Mike Snitzer <snitzer@redhat.com>
>>
>> This patch results in lockdep splats when booting from flash.
>> Reverting it fixes the proboem.
> 
> Should be fixed by:
> https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.15/drivers&id=6e4df4c6488165637b95b9701cc862a42a3836ba
> 

No, it doesn't. I could not apply this patch alone, so I applied the entire series
on top of next-20210813 and gave it another try.

f53c2d11ac98 (HEAD -> master) nbd: reduce the nbd_index_mutex scope
f2f5254b356f nbd: refactor device search and allocation in nbd_genl_connect
d5b03177e069 nbd: return the allocated nbd_device from nbd_dev_add
350b3f6a6e6b nbd: remove nbd_del_disk
49efbeb9de86 nbd: refactor device removal
cdd920eb7cf2 nbd: do del_gendisk() asynchronously for NBD_DESTROY_ON_DISCONNECT
4b358aabb93a (tag: next-20210813, origin/master, origin/HEAD) Add linux-next specific files for 20210813

Still:
...

[   14.467748][    T1]  Possible unsafe locking scenario:
[   14.467748][    T1]
[   14.467928][    T1]        CPU0                    CPU1
[   14.468058][    T1]        ----                    ----
[   14.468187][    T1]   lock(&disk->open_mutex);
[   14.468317][    T1]                                lock(mtd_table_mutex);
[   14.468493][    T1]                                lock(&disk->open_mutex);
[   14.468671][    T1]   lock(mtd_table_mutex);

Guenter