btrfs: Fix a lockdep warning when running xfstest.

The following lockdep warning is triggered during xfstests:

[ 1702.980872] =========================================================
[ 1702.981181] [ INFO: possible irq lock inversion dependency detected ]
[ 1702.981482] 3.18.0-rc1 #27 Not tainted
[ 1702.981781] ---------------------------------------------------------
[ 1702.982095] kswapd0/77 just changed the state of lock:
[ 1702.982415]  (&delayed_node->mutex){+.+.-.}, at: [<ffffffffa03b0b51>] __btrfs_release_delayed_node+0x41/0x1f0 [btrfs]
[ 1702.982794] but this lock took another, RECLAIM_FS-unsafe lock in the past:
[ 1702.983160]  (&fs_info->dev_replace.lock){+.+.+.}

and interrupts could create inverse lock ordering between them.

[ 1702.984675]
other info that might help us debug this:
[ 1702.985524] Chain exists of:
  &delayed_node->mutex --> &found->groups_sem --> &fs_info->dev_replace.lock

[ 1702.986799]  Possible interrupt unsafe locking scenario:

[ 1702.987681]        CPU0                    CPU1
[ 1702.988137]        ----                    ----
[ 1702.988598]   lock(&fs_info->dev_replace.lock);
[ 1702.989069]                                local_irq_disable();
[ 1702.989534]                                lock(&delayed_node->mutex);
[ 1702.990038]                                lock(&found->groups_sem);
[ 1702.990494]   <Interrupt>
[ 1702.990938]     lock(&delayed_node->mutex);
[ 1702.991407]
 *** DEADLOCK ***

It is because the btrfs_kobj_{add/rm}_device() will call memory
allocation with GFP_KERNEL,
which may flush fs page cache to free space, waiting for it self to do
the commit, causing the deadlock.

To solve the problem, move btrfs_kobj_{add/rm}_device() out of the
dev_replace lock range, also involing split the
btrfs_rm_dev_replace_srcdev() function into remove and free parts.

Now only btrfs_rm_dev_replace_remove_srcdev() is called in dev_replace
lock range, and kobj_{add/rm} and btrfs_rm_dev_replace_free_srcdev() are
called out of the lock range.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/dev-replace.c

@@ -577,15 +577,11 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 	list_add(&tgt_device->dev_alloc_list, &fs_info->fs_devices->alloc_list);
 	fs_info->fs_devices->rw_devices++;
 
-	/* replace the sysfs entry */
-	btrfs_kobj_rm_device(fs_info, src_device);
-	btrfs_kobj_add_device(fs_info, tgt_device);
-
 	btrfs_dev_replace_unlock(dev_replace);
 
 	btrfs_rm_dev_replace_blocked(fs_info);
 
-	btrfs_rm_dev_replace_srcdev(fs_info, src_device);
+	btrfs_rm_dev_replace_remove_srcdev(fs_info, src_device);
 
 	btrfs_rm_dev_replace_unblocked(fs_info);
 
@@ -600,6 +596,11 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 	mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 	mutex_unlock(&uuid_mutex);
 
+	/* replace the sysfs entry */
+	btrfs_kobj_rm_device(fs_info, src_device);
+	btrfs_kobj_add_device(fs_info, tgt_device);
+	btrfs_rm_dev_replace_free_srcdev(fs_info, src_device);
+
 	/* write back the superblocks */
 	trans = btrfs_start_transaction(root, 0);
 	if (!IS_ERR(trans))

fs/btrfs/volumes.c

@@ -1800,8 +1800,8 @@ error_undo:
 	goto error_brelse;
 }
 
-void btrfs_rm_dev_replace_srcdev(struct btrfs_fs_info *fs_info,
-				 struct btrfs_device *srcdev)
+void btrfs_rm_dev_replace_remove_srcdev(struct btrfs_fs_info *fs_info,
+					struct btrfs_device *srcdev)
 {
 	struct btrfs_fs_devices *fs_devices;
 
@@ -1829,6 +1829,12 @@ void btrfs_rm_dev_replace_srcdev(struct btrfs_fs_info *fs_info,
 
 	if (srcdev->bdev)
 		fs_devices->open_devices--;
+}
+
+void btrfs_rm_dev_replace_free_srcdev(struct btrfs_fs_info *fs_info,
+				      struct btrfs_device *srcdev)
+{
+	struct btrfs_fs_devices *fs_devices = srcdev->fs_devices;
 
 	call_rcu(&srcdev->rcu, free_device);
 

fs/btrfs/volumes.h

@@ -448,8 +448,10 @@ void btrfs_init_devices_late(struct btrfs_fs_info *fs_info);
 int btrfs_init_dev_stats(struct btrfs_fs_info *fs_info);
 int btrfs_run_dev_stats(struct btrfs_trans_handle *trans,
 			struct btrfs_fs_info *fs_info);
-void btrfs_rm_dev_replace_srcdev(struct btrfs_fs_info *fs_info,
-				 struct btrfs_device *srcdev);
+void btrfs_rm_dev_replace_remove_srcdev(struct btrfs_fs_info *fs_info,
+					struct btrfs_device *srcdev);
+void btrfs_rm_dev_replace_free_srcdev(struct btrfs_fs_info *fs_info,
+				      struct btrfs_device *srcdev);
 void btrfs_destroy_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,
 				      struct btrfs_device *tgtdev);
 void btrfs_init_dev_replace_tgtdev_for_resume(struct btrfs_fs_info *fs_info,