block device read-only handling regression in 2.6.38-rc4 (bisected)

[Milan Broz]

Hi Tejun,

Seems this commit cause regressions:

commit 75f1dc0d076d1c1168f2115f1941ea627d38bd5a
Author: Tejun Heo <tj@xxxxxxxxxx>
Date:   Sat Nov 13 11:55:17 2010 +0100

    block: check bdev_read_only() from blkdev_get()
    
    bdev read-only status can be queried using bdev_read_only() and may
    change while the device is being opened.  Enforce it by checking it
    from blkdev_get() after open succeeds.


1) loop device once set read-only is not able to be used read-write afterward

touch /x1.img
losetup -r /dev/loop0 /x1.img
losetup -d /dev/loop0
losetup /dev/loop0 /x1.img
/dev/loop0: Permission denied


2) it breaks read-only dm-snapshots
(Fedora LiveCD operations is broken by this as well.)

(x.img is backing device, xs.img is prepared COW, you can simply run it
once in read-write to create dm-snap header and then re-run this commands)

losetup -r /dev/loop0 /x.img
losetup -r /dev/loop1 /xs.img
dmsetup create x --readonly --table '0 131072 snapshot /dev/loop0 /dev/loop1 p 8'
device-mapper: reload ioctl failed: Permission denied

Milan
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