FreeRTOS also provides a number of other synchronization mechanisms, including binary semaphores, counting semaphores, and recursive mutexes, that can be used to manage concurrent access to shared resources. The choice of synchronization mechanism depends on the specific requirements of your system and the characteristics of the shared resources being managed.
Mutexes are an essential tool for managing concurrent access to shared resources in FreeRTOS and are widely used in real-time embedded systems, where multiple tasks need to access the same resources in a deterministic and reliable manner. By providing mutual exclusion and preventing race conditions, mutexes help to ensure the correctness and reliability of concurrent systems.
What is mutex in FreeRTOS?
In FreeRTOS, a mutex (short for "mutual exclusion") is a synchronization mechanism that allows multiple tasks to safely share a resource, such as a shared variable or a hardware peripheral, without interfering with each other. A mutex ensures that only one task at a time can access the shared resource, thus preventing concurrent access and potential data corruption or inconsistency.
In FreeRTOS, a mutex is represented by a data structure called SemaphoreHandle_t. A mutex is created using the xSemaphoreCreateMutex() function and can be acquired and released by tasks using the xSemaphoreTake() and xSemaphoreGive() functions, respectively.
How mutex in FreeRTOS works?
A mutex is a synchronization mechanism that provides exclusive access to a shared resource, such as a variable or a hardware peripheral, to prevent data corruption or inconsistency due to concurrent access by multiple tasks.
The basic operation of a mutex involves two main functions: xSemaphoreTake() and xSemaphoreGive(). When a task wants to access a shared resource, it first tries to acquire the mutex by calling xSemaphoreTake(). If the mutex is available, the task acquires the mutex and gains exclusive access to the shared resource. If the mutex is not available because another task already holds it, the calling task is blocked and added to a waiting list.
While a task is holding a mutex, no other task can acquire it. If another task tries to acquire the mutex, it is blocked and added to the waiting list. When the task that is holding the mutex releases it by calling xSemaphoreGive(), the first task waiting on the mutex is unblocked and acquires it, gaining exclusive access to the shared resource. If there are multiple tasks waiting on the mutex, they are unblocked in a first-in, first-out (FIFO) order.
It's important to note that a task must release the mutex it holds before it terminates or blocks for an extended period of time to prevent deadlock, which is a situation where multiple tasks are waiting indefinitely for a mutex that will never be released.
Overall, a mutex provides a simple but effective way to ensure that only one task at a time can access a shared resource, allowing for safe and predictable behavior in concurrent systems.
The Mutex can be explained in the images given below.
The Locked box can be accessed by a single key. Only one person can have the access to the key. Here the Key represents the Mutex and locked box as shared resource. The two tasks are represented by the two persons.
In this image one person gets the key, so he is the owner of the key and he only can access the locked box. When one person has the key then second person has to wait for the same,
In this image ,the first person comes back and handover the key to the second person,Now the second person becomes the owner of the key and can access the locked Box.
What are APIs used for implementing mutex?
In FreeRTOS, there are several APIs available for implementing mutexes. Here are some of the main APIs used for working with mutexes:
xSemaphoreCreateMutex(): This API creates a mutex and returns a handle to it. The handle can be used to manipulate the mutex, such as acquiring or releasing it.
xSemaphoreTake(): This API is used by a task to acquire a mutex. If the mutex is available, the task will acquire it and gain exclusive access to the shared resource. If the mutex is not available, the task will block and be added to a waiting list.
xSemaphoreGive(): This API is used by a task to release a mutex that it is holding. When the mutex is released, the next waiting task will acquire it and gain access to the shared resource.
xSemaphoreGiveFromISR(): This API is used to release a mutex from an interrupt service routine (ISR). It works similarly to xSemaphoreGive(), but is designed to be used in an ISR context.
xSemaphoreTakeRecursive(): This API is used to acquire a mutex in a way that allows the same task to acquire the same mutex multiple times without causing a deadlock. This is sometimes called a recursive mutex.
These APIs provide a simple and effective way to implement mutexes in FreeRTOS, allowing multiple tasks to safely share resources without interfering with each other.
How mutex is different as compared to other semaphores?
Mutexes and semaphores are both synchronization mechanisms used in concurrent programming to control access to shared resources, but they have some key differences.
A semaphore is a variable that is used to signal between tasks, indicating the availability of a shared resource or the completion of a task. There are two types of semaphores: binary semaphores, which can have a value of either 0 or 1, and counting semaphores, which can have a value greater than 1. Tasks can acquire and release semaphores to access shared resources or signal to other tasks.
A mutex is a type of semaphore that is used to provide mutual exclusion to a shared resource. A mutex is a binary semaphore that can only have a value of either 0 or 1, and is used to ensure that only one task can access a shared resource at a time. When a task acquires a mutex, it gains exclusive access to the shared resource, and no other task can access it until the mutex is released.
The main difference between a mutex and other semaphores is that a mutex is designed to provide exclusive access to a shared resource, whereas other semaphores are generally used to signal between tasks or control access to a shared resource with multiple users. A mutex is typically used when a shared resource needs to be accessed by only one task at a time, to prevent data corruption or inconsistencies due to concurrent access.
The mutex is a special type of semaphore that is designed to provide mutual exclusion to a shared resource, whereas other semaphores are generally used for signaling or controlling access to shared resources with multiple users.
Is a binary semaphore is same as mutex?
A binary semaphore is similar to a mutex in that it can be used to provide mutual exclusion to a shared resource, but they are not exactly the same thing.
A binary semaphore is a semaphore that can have a value of either 0 or 1. It is used to signal between tasks, indicating the availability of a shared resource or the completion of a task. Tasks can acquire and release binary semaphores to access shared resources or signal to other tasks. However, unlike a mutex, a binary semaphore does not provide exclusive access to a shared resource, which means that multiple tasks can access the shared resource at the same time, leading to potential data corruption or inconsistencies.
On the other hand, a mutex is a type of semaphore that is used to provide mutual exclusion to a shared resource. When a task acquires a mutex, it gains exclusive access to the shared resource, and no other task can access it until the mutex is released. A mutex is a binary semaphore that can only have a value of either 0 or 1.
A binary semaphore and a mutex are similar in that they are both semaphores, but a mutex is a special type of binary semaphore that provides exclusive access to a shared resource, while a binary semaphore does not provide exclusive access and is used for signaling between tasks or controlling access to a shared resource with multiple users.
What are the advantages of using mutex?
The use of mutexes in concurrent programming provides several advantages, including:
Mutual exclusion: Mutexes provide a mechanism for mutual exclusion, which ensures that only one task can access a shared resource at a time. This prevents data corruption or inconsistencies that can occur when multiple tasks attempt to access the same resource simultaneously.
- Synchronization: Mutexes can be used to synchronize the execution of tasks. By using mutexes, tasks can be made to wait for a shared resource to become available before proceeding with their execution, which helps to ensure that tasks are executed in a specific order.
- Deadlock prevention: Mutexes can be used to prevent deadlocks in concurrent programs. Deadlocks occur when two or more tasks are blocked, waiting for a resource that is held by another task. By using mutexes, tasks can be made to wait for a shared resource to become available, rather than blocking indefinitely and potentially causing a deadlock.
- Priority inversion prevention: Mutexes can be used to prevent priority inversion, which occurs when a high-priority task is blocked by a lower-priority task that is holding a shared resource. By using mutexes, a high-priority task can preempt a low-priority task that is holding a mutex, which helps to prevent priority inversion.
- Increased efficiency: Mutexes can increase the efficiency of concurrent programs by reducing the amount of time that tasks spend waiting for a shared resource to become available. By allowing tasks to access the shared resource in a mutually exclusive manner, the amount of time spent waiting is reduced, which can help to improve the overall performance of the system.
The use of mutexes provides several advantages in concurrent programming, including mutual exclusion, synchronization, deadlock prevention, priority inversion prevention, and increased efficiency.
When to use mutex in FreeRTOS?
Mutexes are used in concurrent programming to protect shared resources from concurrent access by multiple tasks. You should use a mutex in situations where multiple tasks need to access the same shared resource, and it is important to ensure that only one task can access the resource at a time.
Here are some examples of situations where you might want to use a mutex:
- Accessing a shared variable: If multiple tasks need to read or write to the same variable, a mutex can be used to ensure that only one task is accessing the variable at a time. This can help to prevent data corruption or inconsistencies due to concurrent access.
- Accessing shared hardware resources: If multiple tasks need to access the same hardware resource, such as a peripheral device, a mutex can be used to ensure that only one task is accessing the resource at a time. This can help to prevent conflicts and ensure that the hardware is used correctly.
- Implementing critical sections: If you have a section of code that must be executed atomically (i.e., without interruption), you can use a mutex to ensure that only one task is executing the critical section at a time. This can be useful for implementing complex algorithms or data structures that require careful synchronization.
If you have a shared resource that multiple tasks need to access, and it is important to ensure that only one task can access the resource at a time, you should consider using a mutex to provide mutual exclusion.
What are Limitations of mutex?
Although mutexes are a powerful tool for managing concurrent access to shared resources, there are some limitations that you should be aware of:
Priority inversion: If a low-priority task holds a mutex that a high-priority task needs, the high-priority task can be blocked, even though it should have priority. This is known as priority inversion, and it can cause performance problems and even system failures.
Deadlocks: If two or more tasks acquire mutexes in different orders, it is possible for a deadlock to occur, where each task is blocked, waiting for a mutex that is held by another task. Deadlocks can be difficult to detect and resolve, and they can cause the system to hang or crash.
Overhead: Mutexes impose some overhead on the system, both in terms of CPU time and memory usage. The overhead is typically small, but it can become significant if many mutexes are used or if the mutexes are acquired and released frequently.
Complexity: Mutexes can add complexity to a concurrent program, especially if there are many tasks and many shared resources. Managing mutexes and avoiding race conditions can be difficult, and the code can become difficult to understand and maintain.
Risk of misuse: Mutexes can be misused, leading to bugs, crashes, and other problems. For example, if a task forgets to release a mutex, it can cause a deadlock, or if a task acquires a mutex and then blocks indefinitely, it can cause the system to hang.
While mutexes can be a powerful tool for managing concurrent access to shared resources, they also have some limitations and risks that must be taken into account when designing and implementing a concurrent system.
Code Example for implementing Mutex in FreeRTOS
This code uses Esp32 and Arduino IDE to implementing Mutex in FreeRTOS
#if CONFIG_FREERTOS_UNICORE
#define ARDUINO_RUNNING_CORE 0
#else
#define ARDUINO_RUNNING_CORE 1
#endif
SemaphoreHandle_t mutex;
int glbvar = 0;
void task1(void *pvParameters)
{
while (1)
{
if (xSemaphoreTake(mutex, portMAX_DELAY) == pdTRUE)
{
Serial.println("Task 1: Mutex Acquired");
// Access shared resource
glbvar++;
Serial.print("Task 1 - ");
Serial.println(glbvar);
vTaskDelay(1 / portTICK_PERIOD_MS);
xSemaphoreGive(mutex);
Serial.println("Task 1: Mutex Released");
Serial.println("");
Serial.println("");
}
}
}
void task2(void *pvParameters)
{
while (1)
{
if (xSemaphoreTake(mutex, portMAX_DELAY) == pdTRUE)
{
Serial.println("Task 2: Mutex Acquired");
// Access shared resource
glbvar++;
Serial.print("Task 2 - ");
Serial.println(glbvar);
vTaskDelay(1 / portTICK_PERIOD_MS);
xSemaphoreGive(mutex);
Serial.println("Task 2: Mutex Released");
Serial.println("");
Serial.println("");
}
}
}
void setup()
{
Serial.begin(115200);
mutex = xSemaphoreCreateMutex();
xTaskCreatePinnedToCore(task1, "Task 1", 1024, NULL, 1,NULL ,ARDUINO_RUNNING_CORE);
xTaskCreatePinnedToCore(task2, "Task 2", 1024, NULL, 1,NULL, ARDUINO_RUNNING_CORE);
}
void loop()
{
// Do nothing
}
Conclusion
Mutexes are a powerful tool for managing concurrent access to shared resources in FreeRTOS and other real-time operating systems. By providing mutual exclusion, mutexes prevent race conditions and ensure data consistency, which is essential for the correct and reliable operation of concurrent systems.
While mutexes are widely used in real-time embedded systems, they also have some limitations and risks that must be taken into account when designing and implementing a concurrent system. These include priority inversion, deadlocks, overhead, complexity, and the risk of misuse. Therefore, it is important to use mutexes judiciously and to carefully design the concurrent system to avoid these issues.
Related YouTube
FreeRTOS:Using Mutex |ESP32 with Arduino IDE