UBIFS: Oops while rebooting 2.6.34-rc6

[Artem Bityutskiy]

On Fri, 2010-05-07 at 15:16 +0200, Daniel Mack wrote:
> Hi,
> 
> We've had a kernel Oops today when rebooting an ARM PXA based machine
> while file I/O via SSH was outstanding.
> 
> Daniel
> 
> # reboot
> # [  671.190085] UBIFS: un-mount UBI device 0, volume 1
> The system is going down NOW!
> Sent SIGTERM to all processes
> [  672.083833] Unable to handle kernel NULL pointer dereference at virtual address 000000ac
> [  672.094587] pgd = c0004000
> [  672.097301] [000000ac] *pgd=00000000
> [  672.100850] Internal error: Oops: 817 [#1]
> [  672.104919] last sysfs file: /sys/devices/platform/spi_gpio.0/spi0.2/value

It's Firday, and I want to go home, so here is another quick idea for
you where to dig.

When the system reboots it re-mounts the FS to RO mode, usually. And
there is some emergency remount business (see do_emergency_remount()),
which will re-mount the FS even if there are files opened for writing.

So, if there is a UBIFS or VFS bug, and somehow one process is in
make_reservation() and is about to write something, and another process
managed to re-mount the FS to R/O mode, then we may ooops, because UBIFS
frees these 'wbuf' objects when it is mounted to R/O (see
ubifs_remount_ro()).

So, inject printks to ubifs_remount_ro() to check this theory.

Have a nice weekend and bughunting!

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)