linux-kernel - Re: question about bd_inode hashing against device_add() // Re: [PATCH 03/11] block: call bdev_add later in device_add

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Message-ID: <20251031094552.GA10011@lst.de>
Date: Fri, 31 Oct 2025 10:45:52 +0100
From: Christoph Hellwig <hch@....de>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	Jan Kara <jack@...e.cz>, Christian Brauner <brauner@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Luis Chamberlain <mcgrof@...nel.org>, linux-block@...r.kernel.org,
	Joseph Qi <joseph.qi@...ux.alibaba.com>,
	guanghuifeng@...ux.alibaba.com, zongyong.wzy@...baba-inc.com,
	zyfjeff@...ux.alibaba.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: question about bd_inode hashing against device_add() // Re:
 [PATCH 03/11] block: call bdev_add later in device_add_disk

On Fri, Oct 31, 2025 at 05:36:45PM +0800, Gao Xiang wrote:
> Right, sorry yes, disk_uevent(KOBJ_ADD) is in the end.
>
>>  Do you see that earlier, or do you have
>> code busy polling for a node?
>
> Personally I think it will break many userspace programs
> (although I also don't think it's a correct expectation.)

We've had this behavior for a few years, and this is the first report
I've seen.

> After recheck internally, the userspace program logic is:
>   - stat /dev/vdX;
>   - if exists, mount directly;
>   - if non-exists, listen uevent disk_add instead.
>
> Previously, for devtmpfs blkdev files, such stat/mount
> assumption is always valid.

That assumption doesn't seem wrong.  But why does the device node
get created earlier?  My assumption was that it would only be
created by the KOBJ_ADD uevent.  Adding the device model maintainers
as my little dig through the core drivers/base/ code doesn't find
anything to the contrary, but maybe I don't fully understand it.