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

[Christoph Hellwig <hch@lst.de>]

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.