In this lesson, you will learn how to use execution tracing tools to visualize and diagnose race conditions and deadlocks caused by preemptive scheduling in an RTOS.
Visualizing the Scheduler’s Timeline
When your system behaves inconsistently—perhaps a motor jitters or a sensor reading disappears—the culprit is often a scheduling conflict. Because RTOS environments rely on preemptive priority-based scheduling, tasks can be interrupted at any moment to yield to higher-priority processes. If two tasks access a shared resource without proper synchronization, they create a race condition.
To debug this, we use tracing tools (like system viewers or logic analyzers that track Task Control Block transitions). Tracing allows you to capture a high-resolution log of which task was running, when it was preempted, and what system resources were held at the time of the conflict.
Identifying the Conflict
Consider two tasks, Task_A (High Priority) and Task_B (Low Priority), both attempting to modify a global flag.
// Potential Race Condition
void Task_B(void) {
global_flag = 1; // Interruption happens here!
// Task_A preempts Task_B and modifies global_flag
if (global_flag == 1) {
do_critical_action();
}
}
If Task_A runs during the brief window in Task_B, it may change the global_flag unexpectedly. In your trace log, you would see a "Task Context Switch" event occurring exactly between the assignment and the conditional check.
Detecting Deadlocks
A deadlock occurs when Task_A waits for a resource held by Task_B, while Task_B is waiting for a resource held by Task_A. In a trace, this manifests as a "Task Blocked" state that never transitions to "Ready." Unlike a simple race condition, the timeline will show both tasks entering a waiting state and remaining there indefinitely, effectively stalling that branch of your execution pipeline.
By correlating these trace logs with your Task Control Block states, you can pinpoint the exact moment of contention. You can then apply synchronization primitives or adjust task priorities to ensure that critical sections are never interrupted in a way that creates inconsistent states.
Key Takeaway: Use execution tracing to correlate task context switches with resource access; if you see two tasks stuck in a "Blocked" state while awaiting each other, you have identified a deadlock.