[PATCH RFC V5 2/2] btrfs: consolidate device_list_mutex in prepare_sprout to its parent

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

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 <anand.jain@oracle.com>
---
RFC because IMO the cleanup of device_list_mutex makes sense even though
there isn't another thread that could race potentially race as of now.

Depends on
 [PATCH v2] btrfs: fix lockdep warning while mounting sprout fs
which removed the device_list_mutex from clone_fs_devices() otherwise
this patch will cause a double mutex error.

v2: fix the missing mutex_unlock in the error return
v3: -
v4: -
v5: - (Except for the change in below SO comments)

 fs/btrfs/volumes.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index fa9fe47b5b68..53ead67b625c 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2369,6 +2369,8 @@ 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;
 
@@ -2400,7 +2402,6 @@ 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)
@@ -2416,7 +2417,6 @@ 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;
@@ -2591,10 +2591,12 @@ 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);
 		if (ret) {
+			mutex_unlock(&fs_devices->device_list_mutex);
 			btrfs_abort_transaction(trans, ret);
 			goto error_trans;
 		}
@@ -2604,7 +2606,6 @@ 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);
-- 
2.31.1