[PATCH 4/5] btrfs: open block devices after superblock creation

[Christoph Hellwig <hch@lst.de>]

Currently btrfs_mount_root opens the block devices before committing to
allocating a super block. That creates problems for restricting the
number of writers to a device, and also leads to a unusual and not very
helpful holder (the fs_type).

Reorganize the code to first check whether the superblock for a
particular fsid does already exist and open the block devices only if it
doesn't, mirroring the recent changes to the VFS mount helpers.  To do
this the increment of the in_use counter moves out of btrfs_open_devices
and into the only caller in btrfs_mount_root so that it happens before
dropping uuid_mutex around the call to sget.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/btrfs/super.c   | 41 +++++++++++++++++++++++++----------------
 fs/btrfs/volumes.c | 15 +++++----------
 2 files changed, 30 insertions(+), 26 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 7d38f973e991f6..62a933b47e03c3 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1782,7 +1782,6 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 	struct btrfs_fs_info *fs_info = fc->s_fs_info;
 	struct btrfs_fs_context *ctx = fc->fs_private;
 	struct btrfs_fs_devices *fs_devices = NULL;
-	struct block_device *bdev;
 	struct btrfs_device *device;
 	struct super_block *sb;
 	blk_mode_t mode = sb_open_mode(fc->sb_flags);
@@ -1805,15 +1804,8 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 	fs_devices = device->fs_devices;
 	fs_info->fs_devices = fs_devices;
 
-	ret = btrfs_open_devices(fs_devices, mode, &btrfs_fs_type);
+	fs_devices->in_use++;
 	mutex_unlock(&uuid_mutex);
-	if (ret)
-		return ret;
-
-	if (!(fc->sb_flags & SB_RDONLY) && fs_devices->rw_devices == 0)
-		return -EACCES;
-
-	bdev = fs_devices->latest_dev->bdev;
 
 	/*
 	 * From now on the error handling is not straightforward.
@@ -1831,24 +1823,41 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 	set_device_specific_options(fs_info);
 
 	if (sb->s_root) {
-		if ((fc->sb_flags ^ sb->s_flags) & SB_RDONLY)
+		if ((fc->sb_flags ^ sb->s_flags) & SB_RDONLY) {
 			ret = -EBUSY;
+			goto error_deactivate;
+		}
 	} else {
-		snprintf(sb->s_id, sizeof(sb->s_id), "%pg", bdev);
+		struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
+
+		mutex_lock(&uuid_mutex);
+		ret = btrfs_open_devices(fs_devices, mode, &btrfs_fs_type);
+		mutex_unlock(&uuid_mutex);
+		if (ret)
+			goto error_deactivate;
+
+		if (!(fc->sb_flags & SB_RDONLY) && !fs_devices->rw_devices) {
+			ret = -EACCES;
+			goto error_deactivate;
+		}
+
+		snprintf(sb->s_id, sizeof(sb->s_id), "%pg",
+			 fs_devices->latest_dev->bdev);
 		shrinker_debugfs_rename(sb->s_shrink, "sb-btrfs:%s", sb->s_id);
 		btrfs_sb(sb)->bdev_holder = &btrfs_fs_type;
 		ret = btrfs_fill_super(sb, fs_devices, NULL);
-	}
-
-	if (ret) {
-		deactivate_locked_super(sb);
-		return ret;
+		if (ret)
+			goto error_deactivate;
 	}
 
 	btrfs_clear_oneshot_options(fs_info);
 
 	fc->root = dget(sb->s_root);
 	return 0;
+
+error_deactivate:
+	deactivate_locked_super(sb);
+	return ret;
 }
 
 /*
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 65d28c79402091..5ab1b7c2fa3967 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1222,8 +1222,6 @@ static int devid_cmp(void *priv, const struct list_head *a,
 int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 		       blk_mode_t flags, void *holder)
 {
-	int ret;
-
 	lockdep_assert_held(&uuid_mutex);
 	/*
 	 * The device_list_mutex cannot be taken here in case opening the
@@ -1232,14 +1230,11 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 	 * We also don't need the lock here as this is called during mount and
 	 * exclusion is provided by uuid_mutex
 	 */
-	if (!fs_devices->is_open) {
-		list_sort(NULL, &fs_devices->devices, devid_cmp);
-		ret = open_fs_devices(fs_devices, flags, holder);
-		if (ret)
-			return ret;
-	}
-	fs_devices->in_use++;
-	return 0;
+	ASSERT(fs_devices->in_use);
+	if (fs_devices->is_open)
+		return 0;
+	list_sort(NULL, &fs_devices->devices, devid_cmp);
+	return open_fs_devices(fs_devices, flags, holder);
 }
 
 void btrfs_release_disk_super(struct btrfs_super_block *super)
-- 
2.39.2