btrfs: use rcu_barrier() to wait for bdev puts at unmount
Eric Sandeen [Sat, 9 Mar 2013 15:18:39 +0000 (15:18 +0000)]
commit bc178622d40d87e75abc131007342429c9b03351 upstream.

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.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/btrfs/volumes.c

index 3ad7ee9..06744f1 100644 (file)
@@ -591,6 +591,12 @@ int btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
                __btrfs_close_devices(fs_devices);
                free_fs_devices(fs_devices);
        }
+       /*
+        * Wait for rcu kworkers under __btrfs_close_devices
+        * to finish all blkdev_puts so device is really
+        * free when umount is done.
+        */
+       rcu_barrier();
        return ret;
 }