btrfs_prepare_sprout() moves seed devices into its own struct 
fs_devices,
so that its parent function btrfs_init_new_device() can add the 
new sprout
device to fs_info->fs_devices.

Both btrfs_prepare_sprout() and btrfs_init_new_device() needs
device_list_mutex. But they are holding it sequentially, thus 
creates a
small window to an opportunity to race. Close this opportunity 
and hold
device_list_mutex common to both btrfs_init_new_device() and
btrfs_prepare_sprout().

Signed-off-by: Anand Jain <redacted>
---
RFC because I haven't identified the other thread which could 
race with
this, but still does this cleanup makes sense?

 fs/btrfs/volumes.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 91b8422b3f67..f490d1897c56 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c

@@ -2366,6 +2366,8 @@ static int btrfs_prepare_sprout(struct 

btrfs_fs_info *fs_info)
 	u64 super_flags;

 	lockdep_assert_held(&uuid_mutex);
+	lockdep_assert_held(&fs_devices->device_list_mutex);
+

 	if (!fs_devices->seeding)
 		return -EINVAL;

@@ -2397,7 +2399,6 @@ static int btrfs_prepare_sprout(struct 

btrfs_fs_info *fs_info)
 	INIT_LIST_HEAD(&seed_devices->alloc_list);
 	mutex_init(&seed_devices->device_list_mutex);

-	mutex_lock(&fs_devices->device_list_mutex);
 	list_splice_init_rcu(&fs_devices->devices, 
 &seed_devices->devices,
 			      synchronize_rcu);
 	list_for_each_entry(device, &seed_devices->devices, dev_list)

@@ -2413,7 +2414,6 @@ static int btrfs_prepare_sprout(struct 

btrfs_fs_info *fs_info)
 	generate_random_uuid(fs_devices->fsid);
 	memcpy(fs_devices->metadata_uuid, fs_devices->fsid, 
 BTRFS_FSID_SIZE);
 	memcpy(disk_super->fsid, fs_devices->fsid, BTRFS_FSID_SIZE);
-	mutex_unlock(&fs_devices->device_list_mutex);

 	super_flags = btrfs_super_flags(disk_super) &
 		      ~BTRFS_SUPER_FLAG_SEEDING;

@@ -2588,6 +2588,7 @@ int btrfs_init_new_device(struct 

btrfs_fs_info *fs_info, const char *device_path
 	device->dev_stats_valid = 1;
 	set_blocksize(device->bdev, BTRFS_BDEV_BLOCKSIZE);

+	mutex_lock(&fs_devices->device_list_mutex);
 	if (seeding_dev) {
 		btrfs_clear_sb_rdonly(sb);
 		ret = btrfs_prepare_sprout(fs_info);

@@ -2599,7 +2600,6 @@ int btrfs_init_new_device(struct 

btrfs_fs_info *fs_info, const char *device_path

 	device->fs_devices = fs_devices;

-	mutex_lock(&fs_devices->device_list_mutex);
 	mutex_lock(&fs_info->chunk_mutex);
 	list_add_rcu(&device->dev_list, &fs_devices->devices);
 	list_add(&device->dev_alloc_list, &fs_devices->alloc_list);