[PATCH 2.6.35] mtd: blktrans: Move device and module ref-counting back to open/release

[Kevin Cernekee]

On Thu, Jul 22, 2010 at 10:44 AM, Ben Hutchings
<bhutchings at solarflare.com> wrote:
> commit 048d87199566663e4edc4880df3703c04bcf41d9 "mtd: blktrans: Hotplug
> fixes" causes the MTD and the MTD driver module's ref-counts to be
> bumped for each block device that is created, whether or not it's
> actually in use.
>
> This means that before removing an MTD or driver module one must first
> remove all the associated block devices.  But the block devices will
> be removed only when the MTDs themselves are removed, which is no
> longer possible.

Just updated to 2.6.36-rc3 and noticed a related issue.  Configuration is:

CONFIG_MTD_BLOCK=y
CONFIG_MTD_BLKDEVS=y
CONFIG_MTD_UBI_GLUEBI=y

When I "ubiattach" a physical MTD device, a gluebi MTD device is
automatically created for each UBI volume.  The notifiers cause
add_mtd_blktrans_dev() to get called for each new gluebi device.  But
add_mtd_blktrans_dev() now calls __get_mtd_device(), locking up the
gluebi device and preventing it from being used:

# ubiattach -m3
UBI: attaching mtd3 to ubi0
UBI: physical eraseblock size:   131072 bytes (128 KiB)
UBI: logical eraseblock size:    130944 bytes
UBI: smallest flash I/O unit:    1
UBI: VID header offset:          64 (aligned 64)
UBI: data offset:                128
UBI: max. sequence number:       14
UBI: attached mtd3 to ubi0
UBI: MTD device name:            "root"
UBI: MTD device size:            16 MiB
UBI: number of good PEBs:        128
UBI: number of bad PEBs:         0
UBI: max. allowed volumes:       128
UBI: wear-leveling threshold:    4096
UBI: number of internal volumes: 1
UBI: number of user volumes:     1
UBI: available PEBs:             0
UBI: total number of reserved PEBs: 128
UBI: number of PEBs reserved for bad PEB handling: 0
UBI: max/mean erase counter: 1/0
UBI: image sequence number:  0
UBI: background thread "ubi_bgt0d" started, PID 448
UBI device number 0, total 128 LEBs (16760832 bytes, 16.0 MiB),
available 0 LEBs (0 bytes), LEB size 130944 bytes (127.9 KiB)
# mount -t ubifs ubi0:rootfs /mnt
UBI error: ubi_open_volume: cannot open device 0, volume 0, error -16
mount: mounting ubi0:rootfs on /mnt failed: Device or resource busy
# ubidetach -m3
ubidetach: error!: cannot detach mtd3
           error 16 (Device or resource busy)

My short-term workaround is to disable CONFIG_MTD_BLOCK and
CONFIG_MTD_BLKDEVS.  It would be nice to fix the code for 2.6.36
though.