A deadlock occurs when two or more threads are blocked forever, each waiting for the other to release a resource.

The Classic Example

Thread A:                    Thread B:
Lock(resource1)              Lock(resource2)
  ...                          ...
  Lock(resource2) ← waits      Lock(resource1) ← waits
  ...                          ...
Unlock(resource2)            Unlock(resource1)
Unlock(resource1)            Unlock(resource2)

Both threads wait forever.

Four Conditions (Coffman Conditions)

Deadlock requires all four to be true:

Condition Description
Mutual Exclusion Resource can only be held by one thread
Hold and Wait Thread holds resource while waiting for another
No Preemption Resources cannot be forcibly taken
Circular Wait Circular chain of threads waiting

Break any one condition to prevent deadlock.

Prevention Strategies

Strategy How
Lock ordering Always acquire locks in the same order
Lock timeout Give up if lock not acquired in time
Try-lock Non-blocking attempt, back off if fails
Single lock Use one coarse-grained lock
Lock-free structures Avoid locks entirely

Detection

Symptoms:

Tools: