Nobody Is Blocked

This protocol replaces the second blocking acquire with a trylock, and it does remove deadlock: no thread ever holds one lock while waiting on another.

top:
    pthread_mutex_lock(L1);
    if (pthread_mutex_trylock(L2) != 0) {
        pthread_mutex_unlock(L1);
        goto top;
    }
    /* both held: do the work */

Thread 2 runs the same loop with L2 and L1 swapped. What can still go wrong?