1 files changed, 6 insertions, 2 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 34068b887f14..3f1c2c200691 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1620,7 +1620,11 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
         /*
          * the device list mutex makes sure that we don't change
          * the device list while someone else is writing out all
-         * the device supers.
+         * the device supers. Whoever is writing all supers, should
+         * lock the device list mutex before getting the number of
+         * devices in the super block (super_copy). Conversely,
+         * whoever updates the number of devices in the super block
+         * (super_copy) should hold the device list mutex.
          */
 
         cur_devices = device->fs_devices;
@@ -1644,10 +1648,10 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
                 device->fs_devices->open_devices--;
 
         call_rcu(&device->rcu, free_device);
-        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 
         num_devices = btrfs_super_num_devices(root->fs_info->super_copy) - 1;
         btrfs_set_super_num_devices(root->fs_info->super_copy, num_devices);
+        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 
         if (cur_devices->open_devices == 0) {
                 struct btrfs_fs_devices *fs_devices;