From: Eric Sandeen <sandeen@redhat.com>
Date: Sat, 9 Mar 2013 15:18:39 +0000 (+0000)
Subject: btrfs: use rcu_barrier() to wait for bdev puts at unmount
X-Git-Tag: v3.9-rc3~3^2~4
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bc178622d40d87e75abc131007342429c9b03351;p=pandora-kernel.git

btrfs: use rcu_barrier() to wait for bdev puts at unmount

Doing this would reliably fail with -EBUSY for me:

# mount /dev/sdb2 /mnt/scratch; umount /mnt/scratch; mkfs.btrfs -f /dev/sdb2
...
unable to open /dev/sdb2: Device or resource busy

because mkfs.btrfs tries to open the device O_EXCL, and somebody still has it.

Using systemtap to track bdev gets & puts shows a kworker thread doing a
blkdev put after mkfs attempts a get; this is left over from the unmount
path:

btrfs_close_devices
	__btrfs_close_devices
		call_rcu(&device->rcu, free_device);
			free_device
				INIT_WORK(&device->rcu_work, __free_device);
				schedule_work(&device->rcu_work);

so unmount might complete before __free_device fires & does its blkdev_put.

Adding an rcu_barrier() to btrfs_close_devices() causes unmount to wait
until all blkdev_put()s are done, and the device is truly free once
unmount completes.

Cc: stable@vger.kernel.org
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
---

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