Pattern: The Dining Philosophers — Deadlock-Free Coordination (Problem 15)
The canonical deadlock puzzle (Problem 15)
Five philosophers sit in a circle; one fork lies between each pair. To eat, a philosopher needs both adjacent forks. The naive solution — "pick up the left fork, then the right" — deadlocks: if all five grab their left fork at once, every right fork is held by a neighbour, and all five wait forever. It is the four Coffman conditions made physical (see Deadlock).
Three fixes (each breaks one Coffman condition)
- Resource ordering (breaks circular wait): number the forks; always pick up the lower-numbered first. Now one philosopher reaches for the same fork as a neighbour instead of completing the cycle.
- Asymmetry (the simplest): make one philosopher pick up the right fork first. That single reversal cannot close the ring.
- Arbiter / limit to 4 seated (breaks hold-and-wait): a semaphore with 4 permits ensures at least one philosopher can always get both forks.
// Resource-ordering fix (forks are locks 0..4)
Lock lo = forks[min(left,right)], hi = forks[max(left,right)];
lo.lock(); hi.lock();
try { eat(); } finally { hi.unlock(); lo.unlock(); } // always low-then-highIn Go
// limit-to-4 arbiter with a buffered channel as a counting semaphore
seats := make(chan struct{}, 4)
eat := func(l, r *sync.Mutex) {
seats <- struct{}{} // sit (max 4)
l.Lock(); r.Lock()
/* eat */
r.Unlock(); l.Unlock(); <-seats // leave
}Takeaways
- Naive "left then right" = textbook circular-wait deadlock.
- Fix by breaking one Coffman condition: global fork ordering, one asymmetric philosopher, or cap concurrent diners.
- Same lesson as DB lock ordering — consistent acquisition order prevents cycles.
Re-authored for this guide; circular-wait diagram hand-authored as SVG. Covers Problem 15. See also: Deadlock, Livelock & Starvation; Semaphores.
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