From: David Sterba <dsterba@suse.com>
Date: Tue, 20 Mar 2018 23:44:50 +0100
Subject: btrfs: remove wrong use of volume_mutex from btrfs_dev_replace_start
Git-commit: a0fecc23718aa9ef020b8c86173a0b783ed37dcf
Patch-mainline: v4.18-rc1
References:bsc#1154651

The volume mutex does not protect against anything in this case, the
comment about scrub is right but not related to locking and looks
confusing. The comment in btrfs_find_device_missing_or_by_path is wrong
and confusing too.

The device_list_mutex is not held here to protect device lookup, but in
this case device replace cannot run in parallel with device removal (due
to exclusive op protection), so we don't need further locking here.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Acked-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/dev-replace.c |    7 +------
 fs/btrfs/volumes.c     |    4 ----
 2 files changed, 1 insertion(+), 10 deletions(-)

--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -315,18 +315,13 @@ int btrfs_dev_replace_start(struct btrfs
 	struct btrfs_device *tgt_device = NULL;
 	struct btrfs_device *src_device = NULL;

-	/* the disk copy procedure reuses the scrub code */
-	mutex_lock(&fs_info->volume_mutex);
 	ret = btrfs_find_device_by_devspec(fs_info, srcdevid,
 					    srcdev_name, &src_device);
-	if (ret) {
-		mutex_unlock(&fs_info->volume_mutex);
+	if (ret)
 		return ret;
-	}

 	ret = btrfs_init_dev_replace_tgtdev(fs_info, tgtdev_name,
 					    src_device, &tgt_device);
-	mutex_unlock(&fs_info->volume_mutex);
 	if (ret)
 		return ret;

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2133,10 +2133,6 @@ int btrfs_find_device_missing_or_by_path
 		struct btrfs_device *tmp;

 		devices = &fs_info->fs_devices->devices;
-		/*
-		 * It is safe to read the devices since the volume_mutex
-		 * is held by the caller.
-		 */
 		list_for_each_entry(tmp, devices, dev_list) {
 			if (tmp->in_fs_metadata && !tmp->bdev) {
 				*device = tmp;