[PATCH 1/4] btrfs: mark resumed async balance as writing

[Naohiro Aota <naohiro.aota@xxxxxxx>]

When btrfs balance is interrupted with umount, the background balance
resumes on the next mount. There is a potential deadlock with FS freezing
here like as described in commit 26559780b953 ("btrfs: zoned: mark
relocation as writing").

Mark the process as sb_writing. To preserve the order of sb_start_write()
(or mnt_want_write_file()) and btrfs_exclop_start(), call sb_start_write()
at btrfs_resume_balance_async() before taking fs_info->super_lock.

Fixes: 5accdf82ba25 ("fs: Improve filesystem freezing handling")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Naohiro Aota <naohiro.aota@xxxxxxx>
---
 fs/btrfs/volumes.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 1be7cb2f955f..0d27d8d35c7a 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4443,6 +4443,7 @@ static int balance_kthread(void *data)
 	if (fs_info->balance_ctl)
 		ret = btrfs_balance(fs_info, fs_info->balance_ctl, NULL);
 	mutex_unlock(&fs_info->balance_mutex);
+	sb_end_write(fs_info->sb);
 
 	return ret;
 }
@@ -4463,6 +4464,7 @@ int btrfs_resume_balance_async(struct btrfs_fs_info *fs_info)
 		return 0;
 	}
 
+	sb_start_write(fs_info->sb);
 	spin_lock(&fs_info->super_lock);
 	ASSERT(fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE_PAUSED);
 	fs_info->exclusive_operation = BTRFS_EXCLOP_BALANCE;
-- 
2.35.1