Lecture 14: Pthreads Mutex and Condition Variables

In this lecture Pthreads APIs
Mutex
Condition variables 4<br>

Some Pthreads APIs for mutex 5<br>

6 Mutex usage in POSIX Threads pthread_mutex_t m;
pthread_mutex_init(&m);


pthread_mutex_lock(&m);

critical_region();

pthread_mutex_unlock(&m);<br>

pthread_mutex_trylock Return fails when the mutex is already locked
Used for implementing busy waiting 7<br>

Mutex vs condition variables Mutex is good to guarantee mutual exclusion
Allow and block access to the critical regions

Conditional variables
Block threads due to some condition not met 8<br>

9 Condition Variables Allows a thread to wait till a condition is satisfied

Testing the condition must be done within a mutex

With every condition variable, a mutex is associated<br>

Pthreads APIs for condition variables 10<br>

Comparison with Semaphores If a signal is sent to a conditional variable on which no thread is waiting, the signal is lost

Semaphore will accumulate ‘signals’ by up() 11<br>

12 Condition Variables Waiting Thread:

pthread_mutex_lock(&mutex);

while (condition not satisfied) {
pthread_cond_wait( &condition_variable,  &mutex);}

pthread_mutex_unlock(&mutex); Signaling Thread:

pthread_mutex_lock(&mutex);

/* change variable value */

if (condition satisfied) {
pthread_cond_signal( &condition_variable);
}

pthread_mutex_unlock(&mutex);

/* Alternative to cond_signal is
pthread_cond_broadcast( &condition_variable);
*/ pthread_cond_t condition_variable;
pthread_mutex_t mutex;<br>

13 Condition variable and mutex A mutex is passed into wait:pthread_cond_wait(cond_var, mutex)

Mutex is unlocked before the thread sleeps

Mutex is locked again before pthread_cond_wait() returns

Safe to use pthread_cond_wait() in a while loop and check condition again before proceeding<br>

14 Example Usage Write a program using two threads
Thread 1 prints “hello”
Thread 2 prints “world”
Thread 2 should wait till thread 1 finishes before printing

Use a condition variable<br>

15 Using condition variables int thread1_done = 0;
pthread_cond_t cv; pthread_mutex_t mutex; Thread 1:

printf(“hello “);

pthread_mutex_lock(&mutex);
thread1_done = 1;
pthread_cond_signal(&cv);
pthread_mutex_unlock(&mutex); Thread 2:

pthread_mutex_lock(&mutex);

pthread_cond_wait(&cv, &mutex);

printf(“ world\n“);
pthread_mutex_unlock(&mutex); What is the problem in this implementation?<br>

16 Using condition variables int thread1_done = 0;
pthread_cond_t cv; pthread_mutex_t mutex; Thread 1:

printf(“hello “);

pthread_mutex_lock(&mutex);
thread1_done = 1;
pthread_cond_signal(&cv);
pthread_mutex_unlock(&mutex); Thread 2:

pthread_mutex_lock(&mutex);

while (thread1_done == 0) {
pthread_cond_wait(&cv, &mutex);
}

printf(“ world\n“);
pthread_mutex_unlock(&mutex);<br>

17 Multiple Locks proc1( ) {
pthread_mutex_lock(&m1);
/* use object 1 */
pthread_mutex_lock(&m2);

/* use objects 1 and 2 */

pthread_mutex_unlock(&m2);
pthread_mutex_unlock(&m1);
} proc2( ) {
pthread_mutex_lock(&m2);
/* use object 2 */
pthread_mutex_lock(&m1);

/* use objects 1 and 2 */

pthread_mutex_unlock(&m1);
pthread_mutex_unlock(&m2);
} thread a thread b<br>

18 Deadlock<br>