Blockchain Security: Simple Reentrancy Check

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.20;

contract Vault {
    mapping(address => uint256) public balance;
    bool internal locked;

    modifier noReentrancy(){
        require(!locked, "locked");
        locked = true; _; locked = false;
    }

    function deposit() external payable { balance[msg.sender] += msg.value; }

    function withdraw(uint256 amt) external noReentrancy {
        require(balance[msg.sender] >= amt, "low bal");
        balance[msg.sender] -= amt;
        (bool ok,) = msg.sender.call{value: amt}("");
        require(ok, "xfer");
    }
}