Re: commit 108cc873398932af589c295f78c348513b8d70d9 breaks update-grub on btrfs root filesystem

[David Sterba]

On Tue, Sep 08, 2026 at 11:59:43PM +0200, Alex Romosan wrote:
> Please Cc me as I am not subscribed to the list.
> 
> Running my own compiled kernel without initramfs on a lenovo thinkpad
> x1 carbon gen 7.  The linux disk is the only disk on the system. Since
> version 7.3-rc1 i haven't been able to to a grub-update, instead i get
> this error:
> 
> /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?).
> 
> 7.2 is fine. a git-bisect identified commit
> 108cc873398932af589c295f78c348513b8d70d9 as being the culprit.
> reverting this commit from 7.3-rc2 allowed me to run grub-update
> again.
> 
> this is not the first time i reported grub-update being broken on
> btrfs. i reported exactly the same problem on jan 8, 2024
> (https://lkml.iu.edu/hypermail/linux/kernel/2401.1/00596.html). maybe
> the discussion that followed would help come up with a fix that will
> make everybody happy.

I remember debugging that one, https://bugzilla.kernel.org/show_bug.cgi?id=218353
Reverting 108cc8733989 ("btrfs: fix a lockdep caused by path resolution
during device scan") would bring back the lockdep warning and there is a
locking problem.

The commit says it's fixing 2e8b6bc0ab41 ("btrfs: avoid unnecessary
device path update for the same device"), the difference is in lines

(https://bugzilla.suse.com/show_bug.cgi?id=1230641)

-       } else if (!device->name || strcmp(device->name->str, path)) {
+	} else if (!device->name || !is_same_device(device, path)) {

Which gets changed to (by 108cc8733989):

-       } else if (!device->name || !is_same_device(device, path)) {
+       } else if (!device->name || device->devt != path_devt) {

Each change is reaction to a bug, I don't see a clear fix which will
make it work in all cases.