ext4: protect i_disksize update by i_data_sem in direct write path

i_disksize update should be protected by i_data_sem, by either taking
the lock explicitly or by using ext4_update_i_disksize() helper. But the
i_disksize updates in ext4_direct_IO_write() are not protected at all,
which may be racing with i_disksize updates in writeback path in
delalloc buffer write path.

This is found by code inspection, and I didn't hit any i_disksize
corruption due to this bug. Thanks to Jan Kara for catching this bug and
suggesting the fix!
Reported-by: NJan Kara <jack@suse.cz>
Suggested-by: NJan Kara <jack@suse.cz>
Signed-off-by: NEryu Guan <guaneryu@gmail.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/inode.c

@@ -3658,7 +3658,6 @@ static ssize_t ext4_direct_IO_write(struct kiocb *iocb, struct iov_iter *iter)
 {
 	struct file *file = iocb->ki_filp;
 	struct inode *inode = file->f_mapping->host;
-	struct ext4_inode_info *ei = EXT4_I(inode);
 	ssize_t ret;
 	loff_t offset = iocb->ki_pos;
 	size_t count = iov_iter_count(iter);
@@ -3682,7 +3681,7 @@ static ssize_t ext4_direct_IO_write(struct kiocb *iocb, struct iov_iter *iter)
 			goto out;
 		}
 		orphan = 1;
-		ei->i_disksize = inode->i_size;
+		ext4_update_i_disksize(inode, inode->i_size);
 		ext4_journal_stop(handle);
 	}
 
@@ -3790,7 +3789,7 @@ static ssize_t ext4_direct_IO_write(struct kiocb *iocb, struct iov_iter *iter)
 		if (ret > 0) {
 			loff_t end = offset + ret;
 			if (end > inode->i_size) {
-				ei->i_disksize = end;
+				ext4_update_i_disksize(inode, end);
 				i_size_write(inode, end);
 				/*
 				 * We're going to return a positive `ret'