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    Subject[v2.6.34-stable 184/213] btrfs: use rcu_barrier() to wait for bdev puts at unmount
    Date
    From: Eric Sandeen <sandeen@redhat.com>

    -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
    If you see a problem with using this for longterm, please comment.
    -------------------

    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: Paul Gortmaker <paul.gortmaker@windriver.com>
    ---
    fs/btrfs/volumes.c | 6 ++++++
    1 file changed, 6 insertions(+)

    diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
    index c04ebb1e4c98..7f75546941ac 100644
    --- a/fs/btrfs/volumes.c
    +++ b/fs/btrfs/volumes.c
    @@ -565,6 +565,12 @@ 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;
    }

    --
    1.8.5.2


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