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

[Greg Kroah-Hartman]

On Fri, Oct 31, 2025 at 08:23:32PM +0800, Gao Xiang wrote:
> 
> 
> On 2025/10/31 18:12, Gao Xiang wrote:
> > Hi Greg,
> > 
> > On 2025/10/31 17:58, Greg Kroah-Hartman wrote:
> > > On Fri, Oct 31, 2025 at 05:54:10PM +0800, Gao Xiang wrote:
> > > > 
> > > > 
> > > > On 2025/10/31 17:45, Christoph Hellwig wrote:
> 
> ...
> 
> > > > > 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.
> > > > 
> > > > AFAIK, device_add() is used to trigger devtmpfs file
> > > > creation, and it can be observed if frequently
> > > > hotpluging device in the VM and mount.  Currently
> > > > I don't have time slot to build an easy reproducer,
> > > > but I think it's a real issue anyway.
> > > 
> > > As I say above, that's not normal, and you have to be root to do this,
> I just spent time to reproduce with dynamic loop devices and
> actually it's easy if msleep() is located artificiallly,
> the diff as below:
> 
> diff --git a/block/bdev.c b/block/bdev.c
> index 810707cca970..a4273b5ad456 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -821,7 +821,7 @@ struct block_device *blkdev_get_no_open(dev_t dev, bool autoload)
>  	struct inode *inode;
> 
>  	inode = ilookup(blockdev_superblock, dev);
> -	if (!inode && autoload && IS_ENABLED(CONFIG_BLOCK_LEGACY_AUTOLOAD)) {
> +	if (0) {
>  		blk_request_module(dev);
>  		inode = ilookup(blockdev_superblock, dev);
>  		if (inode)
> diff --git a/block/genhd.c b/block/genhd.c
> index 9bbc38d12792..3c9116fdc1ce 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -428,6 +428,8 @@ static void add_disk_final(struct gendisk *disk)
>  	set_bit(GD_ADDED, &disk->state);
>  }
> 
> +#include <linux/delay.h>
> +
>  static int __add_disk(struct device *parent, struct gendisk *disk,
>  		      const struct attribute_group **groups,
>  		      struct fwnode_handle *fwnode)
> @@ -497,6 +499,9 @@ static int __add_disk(struct device *parent, struct gendisk *disk,
>  	if (ret)
>  		goto out_free_ext_minor;
> 
> +	if (disk->major == LOOP_MAJOR)
> +		msleep(2500);           // delay 2.5s for all loops
> +

Yes, so you need to watch for the uevent to happen, THEN it is safe to
access the block device.  Doing it before then isn't a good idea :)

But, if you think this is an issue, do you have a patch that passes your
testing to fix it?

thanks,

greg k-h