4.5 Prevention and Handling Strategies

1. Overcoming Deadlock

   Since detecting and recovering from a deadlock in an embedded system is very difficult, the best approach is prevention. This is done by breaking one of the four Coffman conditions.

   • Break Circular Wait: Enforce a strict lock ordering for all resources. If all tasks are required to lock Mutex 1 before Mutex 2, the deadlock scenario described above could never happen. This is the most common and effective deadlock prevention strategy
   • Break Hold and Wait: Request all required resources at once.
   • Detection and Recovery: Mechanisms like a wait-for graph can be used to detect cycles. If a deadlock is detected, the system can recover by aborting one of the tasks (termination) or forcibly taking a resource (preemption). However, this is rarely implemented on microcontrollers.
     
     

2. Overcoming Priority Inversion and Starvation

   • Priority Inheritance: This is the solution to priority inversion. If a high-priority task is blocked waiting for a mutex held by a low-priority task, the low-priority task will temporarily "inherit" the priority of the high-priority task. This allows the low-priority task to run quickly, finish its work, and release the mutex as soon as possible.

     Note: Mutexes in FreeRTOS already implement this mechanism automatically.

   • Priority Ceiling: Each resource is assigned a priority ceiling, which is the highest priority level of any task that can access it. When a task locks the resource, its priority is immediately raised to that ceiling level until it releases it.

   • Aging: To prevent starvation, the priority of a task that has been waiting for a long time can be gradually increased. This way, a low-priority task will eventually have a high enough priority to be executed.

   • Fair Scheduling: Using a scheduling algorithm (like round-robin for tasks of the same priority) to ensure all tasks get a fair share of CPU time.