btrfs: consolidate device_list_mutex in prepare_sprout to its parent

btrfs_prepare_sprout() splices 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() need
device_list_mutex. But they are holding it separately, thus create a
small race window. Close it and hold device_list_mutex across both
functions btrfs_init_new_device() and btrfs_prepare_sprout().

Split btrfs_prepare_sprout() into btrfs_init_sprout() and
btrfs_setup_sprout(). This split is essential because device_list_mutex
must not be held for allocations in btrfs_init_sprout() but must be held
for btrfs_setup_sprout(). So now a common device_list_mutex can be used
between btrfs_init_new_device() and btrfs_setup_sprout().
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2431,21 +2431,15 @@ struct btrfs_device *btrfs_find_device_by_devspec(
 	return device;
 }
 
-/*
- * does all the dirty work required for changing file system's UUID.
- */
-static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
+static struct btrfs_fs_devices *btrfs_init_sprout(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
 	struct btrfs_fs_devices *old_devices;
 	struct btrfs_fs_devices *seed_devices;
-	struct btrfs_super_block *disk_super = fs_info->super_copy;
-	struct btrfs_device *device;
-	u64 super_flags;
 
 	lockdep_assert_held(&uuid_mutex);
 	if (!fs_devices->seeding)
-		return -EINVAL;
+		return ERR_PTR(-EINVAL);
 
 	/*
 	 * Private copy of the seed devices, anchored at
@@ -2453,7 +2447,7 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
 	 */
 	seed_devices = alloc_fs_devices(NULL, NULL);
 	if (IS_ERR(seed_devices))
-		return PTR_ERR(seed_devices);
+		return seed_devices;
 
 	/*
 	 * It's necessary to retain a copy of the original seed fs_devices in
@@ -2464,7 +2458,7 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
 	old_devices = clone_fs_devices(fs_devices);
 	if (IS_ERR(old_devices)) {
 		kfree(seed_devices);
-		return PTR_ERR(old_devices);
+		return old_devices;
 	}
 
 	list_add(&old_devices->fs_list, &fs_uuids);
@@ -2475,7 +2469,41 @@ 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);
+	return seed_devices;
+}
+
+/*
+ * Splice seed devices into the sprout fs_devices.
+ * Generate a new fsid for the sprouted read-write filesystem.
+ */
+static void btrfs_setup_sprout(struct btrfs_fs_info *fs_info,
+			       struct btrfs_fs_devices *seed_devices)
+{
+	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
+	struct btrfs_super_block *disk_super = fs_info->super_copy;
+	struct btrfs_device *device;
+	u64 super_flags;
+
+	/*
+	 * We are updating the fsid, the thread leading to device_list_add()
+	 * could race, so uuid_mutex is needed.
+	 */
+	lockdep_assert_held(&uuid_mutex);
+
+	/*
+	 * The threads listed below may traverse dev_list but can do that without
+	 * device_list_mutex:
+	 * - All device ops and balance - as we are in btrfs_exclop_start.
+	 * - Various dev_list readers - are using RCU.
+	 * - btrfs_ioctl_fitrim() - is using RCU.
+	 *
+	 * For-read threads as below are using device_list_mutex:
+	 * - Readonly scrub btrfs_scrub_dev()
+	 * - Readonly scrub btrfs_scrub_progress()
+	 * - btrfs_get_dev_stats()
+	 */
+	lockdep_assert_held(&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)
@@ -2491,13 +2519,10 @@ 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;
 	btrfs_set_super_flags(disk_super, super_flags);
-
-	return 0;
 }
 
 /*
@@ -2588,6 +2613,7 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 	struct super_block *sb = fs_info->sb;
 	struct rcu_string *name;
 	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
+	struct btrfs_fs_devices *seed_devices;
 	u64 orig_super_total_bytes;
 	u64 orig_super_num_devices;
 	int ret = 0;
@@ -2671,18 +2697,25 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 
 	if (seeding_dev) {
 		btrfs_clear_sb_rdonly(sb);
-		ret = btrfs_prepare_sprout(fs_info);
-		if (ret) {
+
+		/* GFP_KERNEL allocation must not be under device_list_mutex */
+		seed_devices = btrfs_init_sprout(fs_info);
+		if (IS_ERR(seed_devices)) {
+			ret = PTR_ERR(seed_devices);
 			btrfs_abort_transaction(trans, ret);
 			goto error_trans;
 		}
+	}
+
+	mutex_lock(&fs_devices->device_list_mutex);
+	if (seeding_dev) {
+		btrfs_setup_sprout(fs_info, seed_devices);
 		btrfs_assign_next_active_device(fs_info->fs_devices->latest_dev,
 						device);
 	}
 
 	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);
@@ -2744,7 +2777,7 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 
 		/*
 		 * fs_devices now represents the newly sprouted filesystem and
-		 * its fsid has been changed by btrfs_prepare_sprout
+		 * its fsid has been changed by btrfs_sprout_splice().
 		 */
 		btrfs_sysfs_update_sprout_fsid(fs_devices);
 	}