Man page 9 : sb_start_write

sb_start_write - get write access to a superblock

void sb_start_write(struct super_block *  sb );

sb
    the super we write to

When a process wants to write data or metadata to a file system (i.e. dirty a page or an inode), it should embed the operation in a sb_start_write - sb_end_write pair to get exclusion against file system freezing. This function increments number of writers preventing freezing. If the file system is already frozen, the function waits until the file system is thawed.

Since freeze protection behaves as a lock, users have to preserve ordering of freeze protection and other filesystem locks. Generally, freeze protection should be the outermost lock. In particular, we have:

sb_start_write -> i_mutex (write path, truncate, directory ops, ...) -> s_umount (freeze_super, thaw_super)