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: <20251105-elstern-wippen-3af38a8f40f3@brauner>
Date: Wed, 5 Nov 2025 13:30:24 +0100
From: Christian Brauner <brauner@...nel.org>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>, 
	"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, 
	"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 10:44:53PM +0800, Gao Xiang wrote:
> 
> 
> 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.

If it's easy to do without much of a regression or performance risk then
the device node should only show up once the device is actually ready.
It's certainly best-practive to wait for the uevent though.