The Dining Philosophers, made visible
Five hungry philosophers, five forks, and one little rule that can freeze the whole table forever. Meet locks, mutexes and deadlock — the fun way.
no setup · click things · watch what happensSix tiny ideas. Then you go play with them.
Only one person can hold a fork at a time. If you have it, your neighbour has to wait. That's exactly what a lock does in code: it lets just one task use something at once.
A lock (or mutex — "mutual exclusion") guarantees only one thread enters a critical section at a time.
Picking up a fork = locking. Putting it down = unlocking. While you hold it, nobody else can. When you're done eating, you let go so someone else can grab it.
lock() acquires; unlock() releases. Forgetting to unlock starves everyone else.
If everyone grabs their left fork at the same time, every philosopher is holding one fork and waiting for the other — which their neighbour is holding. Nobody will ever let go. Everyone is stuck forever. That's a deadlock.
Deadlock needs all four at once: mutual exclusion, hold-and-wait, no preemption, and a circular wait. Break any one and you're safe.
Number the forks. Make everyone grab the smaller number first. Now the circle can't close — someone always gets both forks and eats.
A resource hierarchy breaks the circular-wait condition.
A waiter only lets you pick up forks if both are free. So you never get stuck holding just one.
An arbitrator (a single mutex) makes the two pickups atomic, killing hold-and-wait.
Leave one chair empty. With someone always sitting out, there's a gap in the circle, so it can't lock up.
A counting semaphore of N−1 caps concurrent diners and breaks the cycle.
👆 Click a philosopher to pick up a fork