Deadlock and the Lock-Order Rule
Four conditions produce deadlock; break any one. In practice you break circular wait — with a global lock order.
The four conditions
Deadlock requires ALL of: mutual exclusion, hold-and-wait, no preemption, and circular wait. You rarely get to change the first three. The one you control is circular wait.
The lock-order rule
If every thread acquires the same mutexes in the same global order (say, always A before B), no cycle can form — the holder of B never waits for A while someone holds A and waits for B, because nobody acquires B first. Enforce it by convention and by review: the classic production deadlock is two new features acquiring the same two locks in opposite orders.
Escape hatches when order is genuinely hard:
pthread_mutex_trylockon the second lock; if it fails, release and retry — converts deadlock into livelock-or-progress, and livelock is at least diagnosable.- Lock hierarchies with
assert-able levels, checked in debug builds.
The clinic mindset
Diagnosis questions, in order: Which two threads? Which two locks? Who holds what while waiting for what? If you cannot fill in that grid, you do not have a deadlock diagnosis — you have a deadlock suspicion.