btrfs: fix lock dep warning move scratch super outside of chunk_mutex

commit 48b3b9d401ec86899a52003b37331190a35a81a6 upstream.

Move scratch super outside of the chunk lock to avoid below
lockdep warning. The better place to scratch super is in
the function btrfs_rm_dev_replace_free_srcdev() just before
free_device, which is outside of the chunk lock as well.

To reproduce:
  (fresh boot)
  mkfs.btrfs -f -draid5 -mraid5 /dev/sdc /dev/sdd /dev/sde
  mount /dev/sdc /btrfs
  dd if=/dev/zero of=/btrfs/tf1 bs=4096 count=100
  (get devmgt from https://github.com/asj/devmgt.git)
  devmgt detach /dev/sde
  dd if=/dev/zero of=/btrfs/tf1 bs=4096 count=100
  sync
  btrfs replace start -Brf 3 /dev/sdf /btrfs <--
  devmgt attach host7

======================================================
[ INFO: possible circular locking dependency detected ]
4.6.0-rc2asj+ #1 Not tainted
---------------------------------------------------

btrfs/2174 is trying to acquire lock:
(sb_writers){.+.+.+}, at:
[<ffffffff812449b4>] __sb_start_write+0xb4/0xf0

but task is already holding lock:
(&fs_info->chunk_mutex){+.+.+.}, at:
[<ffffffffa05c5f55>] btrfs_dev_replace_finishing+0x145/0x980 [btrfs]

which lock already depends on the new lock.

Chain exists of:
sb_writers --> &fs_devs->device_list_mutex --> &fs_info->chunk_mutex
Possible unsafe locking scenario:
CPU0				CPU1
----				----
lock(&fs_info->chunk_mutex);
				lock(&fs_devs->device_list_mutex);
				lock(&fs_info->chunk_mutex);
lock(sb_writers);

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 839c2e9e89ae97017d02631751b25cdb5667374c..542c04388ba54b33cc1e04e58c54d5ab7fd27305 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1971,11 +1971,8 @@ void btrfs_rm_dev_replace_remove_srcdev(struct btrfs_fs_info *fs_info,
        if (srcdev->missing)
                fs_devices->missing_devices--;
 
-       if (srcdev->writeable) {
+       if (srcdev->writeable)
                fs_devices->rw_devices--;
-               /* zero out the old super if it is writable */
-               btrfs_scratch_superblocks(srcdev->bdev, srcdev->name->str);
-       }
 
        if (srcdev->bdev)
                fs_devices->open_devices--;
@@ -1986,6 +1983,10 @@ void btrfs_rm_dev_replace_free_srcdev(struct btrfs_fs_info *fs_info,
 {
        struct btrfs_fs_devices *fs_devices = srcdev->fs_devices;
 
+       if (srcdev->writeable) {
+               /* zero out the old super if it is writable */
+               btrfs_scratch_superblocks(srcdev->bdev, srcdev->name->str);
+       }
        call_rcu(&srcdev->rcu, free_device);
 
        /*