Re: [PATCH 07/15] block: move blk_unregister_queue() & device_del() after freeze wait

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

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Sender: ming.lei@redhat.com
Subject: Re: [PATCH 07/15] block: move blk_unregister_queue() & device_del() after freeze wait
Message-Id: <Z_3D0PZWolXUdXnK@fedora>
Recipient: yangpanfei@honor.com

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From: Ming Lei <ming.lei@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: "Jens Axboe" <axboe@kernel.dk>, linux-block@vger.kernel.org, "Nilay Shroff" <nilay@linux.ibm.com>, "Shinichiro Kawasaki" <shinichiro.kawasaki@wdc.com>, "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Subject: Re: [PATCH 07/15] block: move blk_unregister_queue() & device_del() after freeze wait
Date: Tue, 15 Apr 2025 10:26:24 +0800
Message-ID: <Z_3D0PZWolXUdXnK@fedora>

On Mon, Apr 14, 2025 at 08:19:10AM +0200, Christoph Hellwig wrote:
> On Thu, Apr 10, 2025 at 09:30:19PM +0800, Ming Lei wrote:
> > Move blk_unregister_queue() & device_del() after freeze wait, and prepare
> > for unifying elevator switch.
> > 
> > This way is just fine, since bdev has been unhashed at the beginning of
> > del_gendisk(), both blk_unregister_queue() & device_del() are dealing
> > with kobject & debugfs thing only.
> 
> While I believe this is the right thing to do, moving the freeze wait
> caused issues in the past, so be careful.  Take a look at:
> 
> commit 4c66a326b5ab784cddd72de07ac5b6210e9e1b06
> Author: Christoph Hellwig <hch@lst.de>
> Date:   Mon Sep 19 16:40:49 2022 +0200
> 
>     Revert "block: freeze the queue earlier in del_gendisk"

Yeah, I know this story.

If it is triggered again with this patch, I will help to root cause.

The last thing we still can do is to just moving blk_unregister_queue()
after queue is frozen, or even elevator tear down. It is allowed to delete
children kobjects after their parent is removed. Just the KOBJ_REMOVE
event need to be ordered.


Thanks,
Ming