Created attachment 421174 [details]
panic log on kernel 2.6.32-27.el6

Description of problem:
After doing online add/remove device test on btrfs, umount btrfs partition takes long time to finish (about 3 minute), and even may cause kernel panic.

There is no useful log when umount btrfs.
I have only panic log on kernel 2.6.32-27

Version-Release number of selected component (if applicable):
Kernel 2.6.32-33.el6.x86_64
btrfs-progs-0.19-10.el6.x86_64

How reproducible:
umount takes long time: 100%
kernel panic: less than 10%

Steps to Reproduce:
mkdir -p /mnt/btrfs
dd if=/dev/null of=/home/img0 bs=1G seek=5
dd if=/dev/null of=/home/img1 bs=1G seek=3
dd if=/dev/null of=/home/img2 bs=1G seek=2
dd if=/dev/null of=/home/img3 bs=1G seek=1
for i in 0 1 2 3;do losetup /dev/loop$i /home/img$i;done

# Create a filesystem across three drives
mkfs.btrfs /dev/loop0 /dev/loop1 /dev/loop2
mkfs.btrfs /dev/loop3
btrfs-show
mount /dev/loop0 /mnt/btrfs
cp -a /etc /mnt/btrfs
cat /mnt/btrfs/etc/fstab
# Add a device
btrfs-vol -a /dev/loop3 /mnt/btrfs
btrfs-show
cat /mnt/btrfs/etc/fstab
# Remove a device
btrfs-vol -r /dev/loop1 /mnt/btrfs
btrfsck /dev/loop0
btrfs-show
cat /mnt/btrfs/etc/fstab

# Clean up
umount /mnt/btrfs
  
Actual results:
umount takes about 3min to finish

Expected results:
umount returns within a reasonable time

Additional info: