[PATCH] use rcu_barrier() to wait for bdev puts at unmount

[Eric Sandeen <sandeen@redhat.com>]

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.

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.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

p.s. I debated putting it into close_ctree(); I don't know if there' anything
else to wait for.  Thoughts?

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 5cbb7f4..258316a 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -680,6 +680,7 @@ int btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
 		__btrfs_close_devices(fs_devices);
 		free_fs_devices(fs_devices);
 	}
+	rcu_barrier();
 	return ret;
 }