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

[Gao Xiang]

On 2025/10/31 22:34, Greg Kroah-Hartman wrote:
> 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?

I just raise it up for some ideas, and this change is
buried into the code refactor and honestly I need to
look into the codebase and related patchsets first.

Currently I have dozens of other development stuffs
on hand, if it's really a regression, I do hope
Christoph or other folks who are familiar with the code
could try to address this.

Thanks,
Gao Xiang

> thanks,
>
> greg k-h