Protocol Stack Error – How to Fix Fatal Error Starting Protocol Stack: Datadir Already Used by Another Process

If you're sure that geth is not already running, check for the presence of a LOCK file in the geth subdirectory of your data directory. Under Ubuntu this will be ~/.ethereum/geth.

I have experienced a rogue LOCK that caused this issue - I'm not sure why, possibly the normal shutdown of the previous geth session was distured.

Remove the Lockfile Debian/Ubuntu

Remove (or rename) ~/.ethereum/geth/LOCK and restart geth.

# Check that geth is not already running:
ps aux | grep geth

# geth not running - Check for presence of LOCK
test -e  && echo LOCK exists || echo LOCK does not exist

# LOCK exists, but geth isn't running so get rid of LOCK
# Rename to be safe - change back if you mess something up
mv ~/.ethereum/geth/LOCK ~/.ethereum/geth/xLOCK

Now try to start geth.

I see two issues, and I think the second is the source of your error:

function createTokens() payable{
    require(msg.value > 0);
    require(_totalSupply.add(tokens) <= maxTokens);
    uint256 tokens = msg.value.mul(RATE);
    balances[msg.sender] = balances[msg.sender].add(tokens);
    _totalSupply = _totalSupply.add(tokens);
    owner.transfer(msg.value);
    require(_totalSupply.add(tokens) <= maxTokens);
}

The line require(_totalSupply.add(tokens) <= maxTokens); occurs twice. Each time is problematic:

1. The first time, tokens has not yet been assigned a value. (I was surprised this even compiled, but apparently variable declarations are hoisted?) So this checks adding 0, which doesn't do what you want.
2. The second time, you've already added tokens to _totalSupply so you're effectively checking "Can I add tokens twice and still stay under maxTokens?"

I would fix the function by moving the earlier require to after the computation of tokens and getting rid of the require at the end:

function createTokens() payable{
    require(msg.value > 0);
    uint256 tokens = msg.value.mul(RATE);
    require(_totalSupply.add(tokens) <= maxTokens);
    balances[msg.sender] = balances[msg.sender].add(tokens);
    _totalSupply = _totalSupply.add(tokens);
    owner.transfer(msg.value);
}