Re: [PATCH -stable] block: destroy bdi before blockdev is unregistered.

From: Jens Axboe
Date: Mon Apr 27 2015 - 12:28:19 EST

On 04/26/2015 10:12 PM, NeilBrown wrote:

Because of the peculiar way that md devices are created (automatically
when the device node is opened), a new device can be created and
registered immediately after the
blk_unregister_region(disk_devt(disk), disk->minors);
call in del_gendisk().

Therefore it is important that all visible artifacts of the previous
device are removed before this call. In particular, the 'bdi'.

commit c4db59d31e39ea067c32163ac961e9c80198fd37
Author: Christoph Hellwig <hch@xxxxxx>
fs: don't reassign dirty inodes to default_backing_dev_info

moved the
call from bdi_unregister() to bdi_destroy() it has been quite easy to
lose a race and have a new (e.g.) "md127" be created after the
blk_unregister_region() call and before bdi_destroy() is ultimately
called by the final 'put_disk', which must come after del_gendisk().

The new device finds that the bdi name is already registered in sysfs
and complains

[ 9627.630029] WARNING: CPU: 18 PID: 3330 at fs/sysfs/dir.c:31 sysfs_warn_dup+0x5a/0x70()
[ 9627.630032] sysfs: cannot create duplicate filename '/devices/virtual/bdi/9:127'

We can fix this by moving the bdi_destroy() call out of
blk_release_queue() (which can happen very late when a refcount
reaches zero) and into blk_cleanup_queue() - which happens exactly when the md
device driver calls it.

Then it is only necessary for md to call blk_cleanup_queue() before
del_gendisk(). As loop.c devices are also created on demand by
opening the device node, we make the same change there.

Fixes: c4db59d31e39ea067c32163ac961e9c80198fd37
Reported-by: Azat Khuzhin <a3at.mail@xxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: stable@xxxxxxxxxxxxxxx (v4.0)
Signed-off-by: NeilBrown <neilb@xxxxxxx>

Hi Jens,
if you could check this and forward on to Linus I'd really appreciate it.

Yup, I added it. BTW, that line needs 3 '-', otherwise git am will pick up the comments below :-)

Thanks Neil.

Jens Axboe

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