btrfs: lift uuid_mutex to callers of btrfs_scan_one_device

Prepartory work to fix race between mount and device scan.

The callers will have to manage the critical section, eg. mount wants to
scan and then call btrfs_open_devices without the ioctl scan walking in
and modifying the fs devices in the meantime.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -915,8 +915,10 @@ static int btrfs_parse_early_options(const char *options, fmode_t flags,
 				error = -ENOMEM;
 				goto out;
 			}
+			mutex_lock(&uuid_mutex);
 			error = btrfs_scan_one_device(device_name,
 					flags, holder, fs_devices);
+			mutex_unlock(&uuid_mutex);
 			kfree(device_name);
 			if (error)
 				goto out;
@@ -1537,7 +1539,9 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
 			return ERR_PTR(error);
 	}
 
+	mutex_lock(&uuid_mutex);
 	error = btrfs_scan_one_device(device_name, mode, fs_type, &fs_devices);
+	mutex_unlock(&uuid_mutex);
 	if (error)
 		goto error_sec_opts;
 
@@ -2232,15 +2236,21 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
 
 	switch (cmd) {
 	case BTRFS_IOC_SCAN_DEV:
+		mutex_lock(&uuid_mutex);
 		ret = btrfs_scan_one_device(vol->name, FMODE_READ,
 					    &btrfs_root_fs_type, &fs_devices);
+		mutex_unlock(&uuid_mutex);
 		break;
 	case BTRFS_IOC_DEVICES_READY:
+		mutex_lock(&uuid_mutex);
 		ret = btrfs_scan_one_device(vol->name, FMODE_READ,
 					    &btrfs_root_fs_type, &fs_devices);
-		if (ret)
+		if (ret) {
+			mutex_unlock(&uuid_mutex);
 			break;
+		}
 		ret = !(fs_devices->num_devices == fs_devices->total_devices);
+		mutex_unlock(&uuid_mutex);
 		break;
 	case BTRFS_IOC_GET_SUPPORTED_FEATURES:
 		ret = btrfs_ioctl_get_supported_features((void __user*)arg);

fs/btrfs/volumes.c

@@ -1224,6 +1224,8 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 	int ret = 0;
 	u64 bytenr;
 
+	lockdep_assert_held(&uuid_mutex);
+
 	/*
 	 * we would like to check all the supers, but that would make
 	 * a btrfs mount succeed after a mkfs from a different FS.
@@ -1242,7 +1244,6 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 		goto error_bdev_put;
 	}
 
-	mutex_lock(&uuid_mutex);
 	device = device_list_add(path, disk_super, &new_device_added);
 	if (IS_ERR(device)) {
 		ret = PTR_ERR(device);
@@ -1251,7 +1252,6 @@ int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 		if (new_device_added)
 			btrfs_free_stale_devices(path, device);
 	}
-	mutex_unlock(&uuid_mutex);
 
 	btrfs_release_disk_super(page);