[PATCH RFC v2 1/4] 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 I haven't identified the other thread which could race with
this, but still does this cleanup makes sense?

v2: fix the missing mutex_unlock in the error return

 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 e7295ec3865e..51cf68785782 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;
@@ -2588,10 +2588,12 @@ 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;
 		}
@@ -2599,7 +2601,6 @@ 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