btrfs: delay btrfs_open_devices() until super block is created

Currently btrfs always calls btrfs_open_devices() before creating the
super block.

It's fine for now because:

- No blk_holder_ops is provided
- btrfs_fs_type is used as a holder

This means no matter who wins the device opening race, the holder will be
the same thus not affecting the later sget_fc() race.

And since no blk_holder_ops is provided, no bdev operation is depending on
the holder.

But this will no longer be true if we want to implement a proper
blk_holder_ops using fs_holder_ops.
This means we will need a proper super block as the bdev holder.

To prepare for such change:

- Add btrfs_fs_devices::holding member
  This will prevent btrfs_free_stale_devices() and btrfs_close_device()
  from deleting the fs_devices when there is another process trying to
  mount the fs.

  Along with the new member, here comes two helpers,
  btrfs_fs_devices_inc_holding() and btrfs_fs_devices_dec_holding().

  This will allow us to hold an fs_devices without opening it.

  This is needed because we can not hold uuid_mutex while calling
  sget_fc(), this will reverse the lock sequence with s_umount, causing
  a lockdep warning.

- Delay btrfs_open_devices() until a super block is returned
  This means we have to hold the initial fs_devices first, then unlock
  uuid_mutex, call sget_fc(), then re-lock uuid_mutex, and decrease the
  holding number.

  For new super block case, we continue to btrfs_open_devices() with
  uuid_mutex hold.
  For existing super block case, we can unlock uuid_mutex and continue.

  Although this means a more complex error handling path, as if we
  didn't call btrfs_open_devices() (either got an existing sb, or
  sget_fc() failed), we can not let btrfs_put_fs_info() to cleanup the
  fs_devices, as it can be freed at any time after we decrease the hold
  on fs_devices and unlock uuid_mutex.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

fs/btrfs/super.c

@@ -1841,7 +1841,6 @@ 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 = btrfs_open_mode(fc);
@@ -1860,23 +1859,38 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 		mutex_unlock(&uuid_mutex);
 		return PTR_ERR(device);
 	}
-
 	fs_devices = device->fs_devices;
+	/*
+	 * We can not hold uuid_mutex calling sget_fc(), it will lead to a
+	 * locking order reversal with s_umount.
+	 *
+	 * So here we increase the holding number of fs_devices, this will ensure
+	 * the fs_devices itself won't be freed.
+	 */
+	btrfs_fs_devices_inc_holding(fs_devices);
 	fs_info->fs_devices = fs_devices;
-
-	ret = btrfs_open_devices(fs_devices, mode, &btrfs_fs_type);
 	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;
 
 	sb = sget_fc(fc, btrfs_fc_test_super, set_anon_super_fc);
-	if (IS_ERR(sb))
+	if (IS_ERR(sb)) {
+		mutex_lock(&uuid_mutex);
+		btrfs_fs_devices_dec_holding(fs_devices);
+		/*
+		 * Since the fs_devices is not opened, it can be freed at any
+		 * time after unlocking uuid_mutex.
+		 * We do not want to double free such dangling fs_devices through
+		 * put_fs_context()->btrfs_free_fs_info().
+		 * So here we reset fs_info->fs_devices to NULL, and let the
+		 * regular fs_devices reclaim path to handle it.
+		 *
+		 * This applies to all later branches where no fs_devices is
+		 * opened.
+		 */
+		fs_info->fs_devices = NULL;
+		mutex_unlock(&uuid_mutex);
 		return PTR_ERR(sb);
+	}
 
 	set_device_specific_options(fs_info);
 
@@ -1888,26 +1902,44 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 		 *
 		 * fc->s_fs_info is not touched and will be later freed by
 		 * put_fs_context() through btrfs_free_fs_context().
-		 *
-		 * And the fs_info->fs_devices will also be closed by
-		 * btrfs_free_fs_context().
 		 */
 		ASSERT(fc->s_fs_info == fs_info);
 
+		mutex_lock(&uuid_mutex);
+		btrfs_fs_devices_dec_holding(fs_devices);
+		fs_info->fs_devices = NULL;
+		mutex_unlock(&uuid_mutex);
 		/*
 		 * At this stage we may have RO flag mismatch between
 		 * fc->sb_flags and sb->s_flags.  Caller should detect such
 		 * mismatch and reconfigure with sb->s_umount rwsem held if
 		 * needed.
 		 */
 	} else {
+		struct block_device *bdev;
+
 		/*
 		 * The first mount of the fs thus a new superblock, fc->s_fs_info
 		 * should be NULL, and the owner ship of our fs_info and fs_devices is
 		 * transferred to the super block.
 		 */
 		ASSERT(fc->s_fs_info == NULL);
 
+		mutex_lock(&uuid_mutex);
+		btrfs_fs_devices_dec_holding(fs_devices);
+		ret = btrfs_open_devices(fs_devices, mode, &btrfs_fs_type);
+		if (ret < 0)
+			fs_info->fs_devices = NULL;
+		mutex_unlock(&uuid_mutex);
+		if (ret < 0) {
+			deactivate_locked_super(sb);
+			return ret;
+		}
+		if (!(fc->sb_flags & SB_RDONLY) && fs_devices->rw_devices == 0) {
+			deactivate_locked_super(sb);
+			return -EACCES;
+		}
+		bdev = fs_devices->latest_dev->bdev;
 		snprintf(sb->s_id, sizeof(sb->s_id), "%pg", bdev);
 		shrinker_debugfs_rename(sb->s_shrink, "sb-btrfs:%s", sb->s_id);
 		btrfs_sb(sb)->bdev_holder = &btrfs_fs_type;

fs/btrfs/volumes.c

@@ -413,6 +413,7 @@ static void free_fs_devices(struct btrfs_fs_devices *fs_devices)
 	struct btrfs_device *device;
 
 	WARN_ON(fs_devices->opened);
+	WARN_ON(fs_devices->holding);
 	while (!list_empty(&fs_devices->devices)) {
 		device = list_first_entry(&fs_devices->devices,
 					  struct btrfs_device, dev_list);
@@ -540,7 +541,7 @@ static int btrfs_free_stale_devices(dev_t devt, struct btrfs_device *skip_device
 				continue;
 			if (devt && devt != device->devt)
 				continue;
-			if (fs_devices->opened) {
+			if (fs_devices->opened || fs_devices->holding) {
 				if (devt)
 					ret = -EBUSY;
 				break;
@@ -1196,7 +1197,7 @@ void btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
 
 	mutex_lock(&uuid_mutex);
 	close_fs_devices(fs_devices);
-	if (!fs_devices->opened) {
+	if (!fs_devices->opened && !fs_devices->holding) {
 		list_splice_init(&fs_devices->seed_list, &list);
 
 		/*

fs/btrfs/volumes.h

@@ -422,6 +422,17 @@ struct btrfs_fs_devices {
 	/* Count fs-devices opened. */
 	int opened;
 
+	/*
+	 * Counter of the processes that are holding this fs_devices but not
+	 * yet opened.
+	 * This is for mounting handling, as we can only open the fs_devices
+	 * after a super block is created.
+	 * But we can not hold uuid_mutex during sget_fc(), thus we have to
+	 * hold the fs_devices (meaning it can not be released) until a super
+	 * block is returned.
+	 */
+	int holding;
+
 	/* Set when we find or add a device that doesn't have the nonrot flag set. */
 	bool rotating;
 	/* Devices support TRIM/discard commands. */
@@ -854,6 +865,20 @@ static inline void btrfs_warn_unknown_chunk_allocation(enum btrfs_chunk_allocati
 	WARN_ONCE(1, "unknown allocation policy %d, fallback to regular", pol);
 }
 
+static inline void btrfs_fs_devices_inc_holding(struct btrfs_fs_devices *fs_devices)
+{
+	lockdep_assert_held(&uuid_mutex);
+	ASSERT(fs_devices->holding >= 0);
+	fs_devices->holding++;
+}
+
+static inline void btrfs_fs_devices_dec_holding(struct btrfs_fs_devices *fs_devices)
+{
+	lockdep_assert_held(&uuid_mutex);
+	ASSERT(fs_devices->holding > 0);
+	fs_devices->holding--;
+}
+
 void btrfs_commit_device_sizes(struct btrfs_transaction *trans);
 
 struct list_head * __attribute_const__ btrfs_get_fs_uuids(void);