If you do the following

mkfs.btrfs /dev/sdb /dev/sdc
rmmod btrfs
dd if=/dev/zero of=/dev/sdb bs=1M count=1
mount -o degraded /dev/sdc /mnt/btrfs-test

the box will panic trying to deref the name for the missing dev since it is
the lower numbered devid.  So fix show_devname to not use missing devices.
Thanks,

Signed-off-by: Josef Bacik <[email protected]>
---
 fs/btrfs/super.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index fa61ef5..ffcfe49 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1505,6 +1505,8 @@ static int btrfs_show_devname(struct seq_file *m, struct 
dentry *root)
        while (cur_devices) {
                head = &cur_devices->devices;
                list_for_each_entry(dev, head, dev_list) {
+                       if (dev->missing)
+                               continue;
                        if (!first_dev || dev->devid < first_dev->devid)
                                first_dev = dev;
                }
-- 
1.7.7.6

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