Question

I’m actually writing an MPI program. This is a basic client / server pattern. The server have a set of work to compute. The clients get subsets of this big set. Each client use several threads to compute the subset. I must be sure all the threads finished before requesting another subset to the server.

The client is split into several threads: a master (for the communications) and several workers.

Here my actual code, where the deadlock should occur. I cleaned it of any MPI calls, to make it more readable. I really can not see what I am doing wrong here.

void* worker ( void * arg ) {   while (work != 0) {     int x;     while ( get_job(&x) ) {       do_job(x);     }     pthread_mutex_lock(&mutex_worker);     nb_job_empty++;     if (nb_job_empty == NB_THREADS) {       nb_job_empty = 0;       pthread_cond_signal(&cond_master);     }     nb_worker_woken--;     pthread_cond_wait(&cond_worker, &mutex_worker);     nb_worker_woken++;     pthread_mutex_unlock(&mutex_worker);   }     return ( void * ) 0 ; }  void server() {     for (int i = 0; i < 100; i++) {       add_job();       pthread_mutex_lock(&mutex_master);       pthread_cond_broadcast(&cond_worker);       pthread_cond_wait(&cond_master, &mutex_master);       pthread_mutex_unlock(&mutex_master);     }     work = 0;     pthread_mutex_lock(&mutex_master);     pthread_cond_broadcast(&cond_worker);     pthread_mutex_unlock(&mutex_master); } 

Answer 1

You should use the same mutex for both condition (mutex_master and mutex_worker should be the same)

Moreover, you should use pthread_cond_wait in a while loop as it is recommended 🙂