Synchronization and mutual exclusion mechanism in Linux

In a multi-process or multi-thread operating system environment, synchronization and mutual exclusion are key concepts used to ensure correct access to shared resources. The following are the design principles of synchronization and mutual exclusion as well as

Implementation in Linux:

Synchronization

Synchronization mechanism is a process that coordinates the execution of multiple execution threads or processes to ensure that they execute in a certain order or wait under specific conditions. Common synchronization mechanisms include semaphores, condition variables, barriers, etc.

Design Principle

1. Atomic Operations: Atomic operations refer to indivisible operations, either all of them are executed or none of them are executed. In synchronization, atomic operations are an essential element to ensure safe execution of a thread or process.

2. Mutual Exclusion: A key goal of synchronization is to ensure mutually exclusive access to shared resources, that is, only one thread or process can access shared resources at the same time to avoid race conditions.

3. Condition Waiting: Synchronization mechanisms usually need to support conditional waiting, that is, a thread or process waits before a certain condition is met, and other threads or processes notify the waiting thread when the condition is met. Execution continues to achieve coordination between threads.

4. Order Preservation: Synchronization may also involve control of the order of execution to ensure that threads or processes execute in the expected order, thereby ensuring the correctness and reliability of the program.

Implementation in Linux

• Semaphore: Through semaphores, resources can be counted to ensure that only a limited number of threads or processes can access shared resources at the same time. In Linux, semaphores are typically operated using functions such as sem_init, sem_wait, and sem_post.
• Condition variables: Condition variables allow threads to wait before a certain condition is met, and to be notified to continue execution when the condition is met. In Linux, condition variables are typically manipulated using functions such as pthread_cond_init, pthread_cond_wait, and pthread_cond_signal.

Mutex(Mutex)

Mutual exclusion is a mechanism used to ensure mutually exclusive access to shared resources. In a multi-threaded or multi-process environment, mutex locks are the most common mutual exclusion mechanism.

Design Principle

1. Mutex lock: A mutex lock is a lock used to ensure that only one thread can access shared resources at the same time. When one thread acquires the mutex lock, other threads must wait.
2. Critical Section: The critical section is a section of code that may access shared resources, and only one thread can enter at the same time. Mutex locks are often used to protect critical sections.
3. Deadlock avoidance: When designing a mutex mechanism, you need to consider deadlock avoidance to ensure that the system does not fall into an unrelieved wait due to the use of mutex locks.

Implementation in Linux

• Mutex (Mutex): In Linux, mutex locks are usually operated through functions such as pthread_mutex_init, pthread_mutex_lock and pthread_mutex_unlock. They allow threads to enter and exit critical sections safely.
• Spinlock: A spin lock is a lock that does not give up the CPU but keeps checking in a loop while waiting for a mutex lock.In Linux, spin locks are usually operated via spin_lock and spin_unlock.

The above are some common mechanisms to achieve synchronization and mutual exclusion in Linux. The specific choice depends on the needs of the application and the trade-offs between performance and maintainability.

In the following sample code, I will show the use of mutex (Mutex) and condition variable (Condition Variable) to implement a simple synchronization mechanism. The related functions of the POSIX thread library are used here.

#include
#include
#include

#define BUFFER_SIZE 5

int buffer[BUFFER_SIZE];
int count = 0;

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond_producer = PTHREAD_COND_INITIALIZER;
pthread_cond_t cond_consumer = PTHREAD_COND_INITIALIZER;

void *producer(void *arg) {
for (int i = 0; i while (count == BUFFER_SIZE) {
                 // Buffer is full, waiting for consumer consumption
                 pthread_cond_wait(&cond_producer, &mutex);
}

       buffer[count] = i;
​​​​printf("Produced: %d\n", i);

             // Notify consumers that they can consume
           pthread_cond_signal(&cond_consumer);
          pthread_mutex_unlock(&mutex);
}

pthread_exit(NULL);
}

void *consumer(void *arg) {
for (int i = 0; i while (count == 0) {
                   // Buffer is empty, waiting for the producer to produce
                 pthread_cond_wait(&cond_consumer, &mutex);
}

          int item = buffer[--count];
​​​​printf("Consumed: %d\n", item);

              // Notify the producer that production is ready
           pthread_cond_signal(&cond_producer);
          pthread_mutex_unlock(&mutex);
}

pthread_exit(NULL);
}

int main() {
pthread_t producer_thread, consumer_thread;

//Create producer and consumer threads
pthread_create(&producer_thread, NULL, producer, NULL);
pthread_create(&consumer_thread, NULL, consumer, NULL);

// Wait for the thread to end
pthread_join(producer_thread, NULL);
pthread_join(consumer_thread, NULL);

// Destroy mutex lock and condition variable
pthread_mutex_destroy(&mutex);
​​pthread_cond_destroy(&cond_producer);
​​pthread_cond_destroy(&cond_consumer);

Return 0;
}

This simple example demonstrates a producer-consumer problem, in which the producer thread is responsible for producing data into the buffer, and the consumer thread is responsible for consuming data from the buffer. The mutex lock mutex is used to ensure exclusive access to the shared resource, while the condition variables cond_producer and cond_consumer are used to wait when the buffer is full or empty and notifications.

Please note that synchronization and mutual exclusion in actual applications may be more complex, and the specific design depends on the needs of the application.

The following is a simple sample code that demonstrates how to use pthread_mutex_t in Linux to implement a mutex lock. In this example, two threads share a counter, and a mutex lock ensures mutually exclusive access to the counter.

#include
#include

// Shared counter
int counter = 0;

// Mutex lock
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

//Thread function, increase the value of the counter
void* increment_counter(void* arg) {
for (int i = 0; i main() {
//Create two threads
pthread_t thread1, thread2;
pthread_create(&thread1, NULL, increment_counter, NULL);
pthread_create(&thread2, NULL, increment_counter, NULL);

// Wait for the thread to end
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);

// Destroy the mutex lock
pthread_mutex_destroy(&mutex);

// Output the final counter value
Printf("Final Counter Value: %d\n", counter);

Return 0;
}

In this example, two threads concurrently increment the value of the counter variable. Since both threads share the same variable, a race condition exists. Mutex lock mutex is used to ensure mutually exclusive access to counter. One thread first locks the lock when accessing counter, and then unlocks it after completion, so that the other thread thread can enter.

To use a mutex lock, you need to pay attention to the following points:

1. Initialize the mutex lock: Use PTHREAD_MUTEX_INITIALIZER or pthread_mutex_init to initialize the mutex lock.
2. Locking and unlocking: Use pthread_mutex_lock to lock and pthread_mutex_unlock to unlock. Access to shared resources within a critical section should be between locking and unlocking.
3. Destroy the mutex lock: Use pthread_mutex_destroy to destroy the mutex lock when it is no longer needed.

The above code demonstrates how to use a mutex lock to ensure safe access to shared resources and prevent race conditions.

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