[PATCH v2 1/3] btrfs: scrub: maintain the unlock order in scrub thread

[Anand Jain <anand.jain@oracle.com>]

The device_list_mutex and scrub_lock creates a nested locks in
btrfs_scrub_dev().

During lock the order is device_list_mutex and then scrub_lock, and during
unlock, the order is device_list_mutex and then scrub_lock.
Fix this to the lock order of scrub_lock and then device_list_mutex.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v1->v2: change the order of lock acquire first scrub_lock and then
	device_list_mutex, which matches with the order of unlock.
	The extra line which are now in the scrub_lock are ok to be
	under the scrub_lock.
 fs/btrfs/scrub.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 902819d3cf41..a9d6fc3b01d4 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3813,28 +3813,29 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		return -EINVAL;
 	}
 
-
+	mutex_lock(&fs_info->scrub_lock);
 	mutex_lock(&fs_info->fs_devices->device_list_mutex);
 	dev = btrfs_find_device(fs_info, devid, NULL, NULL);
 	if (!dev || (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state) &&
 		     !is_dev_replace)) {
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		mutex_unlock(&fs_info->scrub_lock);
 		return -ENODEV;
 	}
 
 	if (!is_dev_replace && !readonly &&
 	    !test_bit(BTRFS_DEV_STATE_WRITEABLE, &dev->dev_state)) {
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		mutex_unlock(&fs_info->scrub_lock);
 		btrfs_err_in_rcu(fs_info, "scrub: device %s is not writable",
 				rcu_str_deref(dev->name));
 		return -EROFS;
 	}
 
-	mutex_lock(&fs_info->scrub_lock);
 	if (!test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &dev->dev_state) ||
 	    test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &dev->dev_state)) {
-		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		mutex_unlock(&fs_info->scrub_lock);
 		return -EIO;
 	}
 
@@ -3843,23 +3844,23 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	    (!is_dev_replace &&
 	     btrfs_dev_replace_is_ongoing(&fs_info->dev_replace))) {
 		btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
-		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		mutex_unlock(&fs_info->scrub_lock);
 		return -EINPROGRESS;
 	}
 	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
 
 	ret = scrub_workers_get(fs_info, is_dev_replace);
 	if (ret) {
-		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		mutex_unlock(&fs_info->scrub_lock);
 		return ret;
 	}
 
 	sctx = scrub_setup_ctx(dev, is_dev_replace);
 	if (IS_ERR(sctx)) {
-		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		mutex_unlock(&fs_info->scrub_lock);
 		scrub_workers_put(fs_info);
 		return PTR_ERR(sctx);
 	}
-- 
1.8.3.1