race data vs race condition

data-race-n-race-condition.md

Race data example

The result is 11, 12, or 13

// balance = 10
public void deposit(int amount) {
  balance += amount;
}

thread1 -> deposit(1)
thread2 -> deposit(2)

Not race condition example

If current balance is 100, then not race condition. The result is 90

// balance = 100
public synchronized void deposit(int amount) {
  balance += amount;
}

public synchronized void withdraw(int amount) {
  if (balance >= amount) {
    balance -= amount;
  }
}

thread1 -> deposit(10)
thread2 -> withdraw(20)

race condition example

But when the current balance is 10. Then:

We don't know the final result is, it maybe 0 (deposit success, withdraw success) or 20 (withdraw fail, deposit success)

// balance = 10

public synchronized void deposit(int amount) {
  balance += amount;
}

public synchronized void withdraw(int amount) {
  if (balance >= amount) {
    balance -= amount;
  }
}

thread1 -> deposit(10)
thread2 -> withdraw(20)

How to prevent race condition

Queue is not a solution, althought items in queue are ordered, but the requests before pushed into queue are not ordered.

How use solve it?

• Using a priority queue (min-heap), the compare strategy is:
  • Compare pushed time.
  • Compare hashCode or something else