Understanding Concurrency and Critical Sections
In a multi-threaded RTOS (Real-Time Operating System) environment, concurrency refers to the management of multiple tasks or threads that can be executed simultaneously. Each thread in a multi-threaded application shares the same memory address space, which can lead to issues when multiple threads try to read/write shared resources concurrently. Key areas to focus on include critical sections — portions of code that access shared resources and need synchronized access to ensure data integrity.
Mutexes for Mutual Exclusion
A mutex (mutual exclusion) is a lock that allows only one thread to access a critical section at a time. To use a mutex, you need to wrap the critical section with lock and unlock operations.
#include <pthread.h>
pthread_mutex_t myMutex;
void shared_function() {
pthread_mutex_lock(&myMutex);
// Critical section
// Access shared resources here
pthread_mutex_unlock(&myMutex);
}
void setup() {
// Initialize the mutex before use
pthread_mutex_init(&myMutex, NULL);
}
Using mutexes prevents race conditions by ensuring that only one thread can execute a block of code at once.
Semaphores for Synchronization
Semaphores are more flexible than mutexes and can be used to control access to a resource pool.
#include <semaphore.h>
sem_t mySemaphore;
void producer() {
// Produce an item
// Signal that a new item has been produced
sem_post(&mySemaphore);
}
void consumer() {
sem_wait(&mySemaphore);
// Consume an item
}
void setup() {
// Initialize semaphore with 0 indicating unavailable resource
sem_init(&mySemaphore, 0, 0);
}
Semaphores allow more complex synchronization between threads, including counting semaphores for managing a finite number of resources.
Spinlocks for Low-latency
Spinlocks are another approach for achieving mutual exclusion, useful in low-latency environments. However, they are CPU-intensive as a thread keeps checking for the lock availability.
#include <pthread.h>
pthread_spinlock_t spinlock;
void setup() {
pthread_spin_init(&spinlock, PTHREAD_PROCESS_PRIVATE);
}
void worker() {
pthread_spin_lock(&spinlock);
// Critical section
pthread_spin_unlock(&spinlock);
}
Spinlocks are beneficial in scenarios where locks are expected to be held for a very short time as they avoid the overhead of waking a sleeping thread.
Atomic Variables for Non-blocking Synchronization
For operations on single variables, consider using atomic operations, which are lock-free and provide non-blocking synchronization.
#include <stdatomic.h>
atomic_int counter = 0;
void increment_counter() {
atomic_fetch_add(&counter, 1);
}
Atomic operations can improve performance by avoiding locks when only simple operations on a variable are required.
Thread Priorities and Scheduling
In RTOS environments, setting thread priorities is crucial. High-priority threads can preempt lower-priority ones to meet real-time constraints.
#include <pthread.h>
void setup_thread(pthread_t *thread, void *(*function)(void *)) {
pthread_attr_t attr;
pthread_attr_init(&attr);
struct sched_param param;
param.sched_priority = 10; // Set priority
pthread_attr_setschedparam(&attr, ¶m);
pthread_create(thread, &attr, function, NULL);
}
Ensure your RTOS supports priority scheduling and utilize it to meet deadlines and respond quickly to real-time events.
Avoiding Priority Inversion
Priority inversion happens when a lower-priority thread holds a lock needed by a higher-priority thread. Utilize priority inheritance protocols if supported by your RTOS to mitigate this issue.
// Assuming RTOS has priority inheritance
pthread_mutex_t mutex;
pthread_mutexattr_t mta;
pthread_mutexattr_init(&mta);
pthread_mutexattr_setprotocol(&mta, PTHREAD_PRIO_INHERIT);
pthread_mutex_init(&mutex, &mta);
Priority inheritance temporarily boosts the priority of the thread holding the lock to match that of the highest-priority blocked thread.
Each concurrency handling mechanism serves different requirements and may be utilized based on the system's constraints and behavior. Use these tools judiciously to maintain a robust and efficient multi-threaded environment in your firmware development.
