Re: Proper error-handling of cdev_add() after cdev_alloc() ?

[Philipp Matthias Hahn]

Hello,

> How should errors of `cdev_add()` after `cdev_alloc()` be handled?

sorry for ping-ing you directly, but so far got no answer and I would
like to get a definitive answer on this before I spend more time on this
to create and send patches. Thank you.

Am Tue, May 12, 2026 at 10:20:13AM +0200 schrieb Philipp Matthias Hahn:
> The basic schema seems to be like this:
> 
> 	cdev = cdev_alloc();
> 	if (!cdev)
> 		return -ENOMEM;
> 	...
> 	ret = cdev_add(cdev, ...);
> 	if (ret < 0) {
> 		/* WHAT HERE ??? */
> 		return ret;
> 	}
> 	...
> 	return 0;  // success
> 
> I found 5 case of how errors are handled:
> 
> 1.  kfree(cdev);
> 	drivers/media/v4l2-core/v4l2-dev.c:1060
> 
> 2.  kobject_put(&cdev->kobj);
> 	drivers/char/xillybus/xillybus_class.c:107
> 	fs/char_dev.c:293
> 	drivers/s390/char/vmlogrdr.c:780
> 	drivers/tty/tty_io.c:3171
> 	drivers/uio/uio.c:925
> 
> 3.  cdev_del(cdev);
> 	drivers/char/virtio_console.c:1435
> 	fs/fuse/cuse.c:419
> 	drivers/misc/mei/main.c:1318
> 	drivers/s390/char/tape_class.c:92
> 	drivers/s390/char/vmur.c:928
> 	drivers/scsi/sg.c:1560
> 	drivers/scsi/st.c:4307
> 	drivers/staging/vme_user/vme_user.c:694
> 
> 4.  No error handling at all.
> 	arch/sh/boards/mach-landisk/gio.c:145
> 
> 5.  panic()
> 
> Currently I'm unsure of what's the correct way.
> My current understanding is, that `cdev_del()` should only be called if
> `cdev_add()` succeeded, so those findings above should be fixed. But
> to what?

I guess __register_chrdev() from fs/char_dev.c:293 is correct and
	kobject_put(&cdev->kobj);
should be used.

Thank you in advance
Philipp