How to Fix Deadlocks: Step-by-Step Guide

Understand Deadlocks

• Identify sections of the code where multiple threads acquire locks. A deadlock occurs when two or more threads wait indefinitely for a set of locks held by each other.
• Be aware of the four conditions that can lead to a deadlock: mutual exclusion, hold and wait, no preemption, and circular wait.

Use Lock Hierarchy

• Establish a strict ordering of lock acquisition. Ensure every part of the program adheres to this ordering to prevent circular wait.
• Example: If multiple locks need to be acquired (Lock A then Lock B), enforce that ordering everywhere in your code.

• pthread_mutex_lock(&lockA);
  pthread_mutex_lock(&lockB);
  /_ Perform critical section actions _/
  pthread_mutex_unlock(&lockB);
  pthread_mutex_unlock(&lockA);
  

Timeout and Retry

• Implement timeout for lock acquisition. If a lock cannot be acquired within a certain time, release any other locks already held and retry.
• This approach can help break circular waits.

• struct timespec timeout;
  clock_gettime(CLOCK_REALTIME, &timeout);
  timeout.tv\_sec += 1; // wait for 1 second
  
  if (pthread_mutex_timedlock(&lockA, &timeout) == 0) {
    if (pthread_mutex_timedlock(&lockB, &timeout) == 0) {
      /_ Perform critical section actions _/
      pthread_mutex_unlock(&lockB);
    }
    pthread_mutex_unlock(&lockA);
  }
  

  
  

Use Try-locks

• Attempt to acquire locks using non-blocking variants like try-lock. If unable to acquire all required locks, release those acquired and try again later.
• This reduces the chances of contributing to a deadlock.

• if (pthread_mutex_trylock(&lockA) == 0) {
    if (pthread_mutex_trylock(&lockB) == 0) {
        /_ Perform critical section actions _/
        pthread_mutex_unlock(&lockB);
    } else {
        pthread_mutex_unlock(&lockA);
    }
  }
  

Use Deadlock Detection

• Implement a mechanism to detect deadlocks at runtime. This involves periodically checking for threads in waiting states due to lock acquisition failures.
• If detected, attempt recovery through preemption or by breaking the circular wait.

Minimize Lock Usage

• Reduce the scope and number of locks. Smaller critical sections decrease contention and likelihood of deadlocks.
• Consider single lock designs (coarse-grained locks) where feasible, as they simplify lock management.

Analyze with Debug Tools

• Utilize tools like thread analyzers or debuggers to trace lock acquisition order and identify potential deadlocks in your firmware.
• Tools specific to an OS or environment can automatically check and alert about possible deadlock situations.