Multi threaded C program freezes when I change number of threads - c

Question

I am writing a multi threaded c program to multiply two matrices and find the row norm using pthreads and Blas. I thought I had it working when I set the dimension of the matrices to 4 and the number of threads to use to 2. I then changed the number of threads, and it no longer works. It does not compute the wrong answers, but gets stuck when I try to join the threads
void *matrix_norm(void *arg){
mat_norm_t *thread_mat_norm_data = arg;
int n = thread_mat_norm_data->n;
int i, j;
double norm = 0.;
for(i=0;i<thread_mat_norm_data->sub_n;i++){
double row_sum = 0.;
for(j=0;j<n;j++){
row_sum += *(thread_mat_norm_data->z+i*n+j);
}
if(row_sum>norm){
norm = row_sum;
}
}
pthread_mutex_lock(thread_mat_norm_data->mutex);
if (norm > *(thread_mat_norm_data->global_norm)){
*(thread_mat_norm_data->global_norm)=norm;
}
pthread_mutex_unlock(thread_mat_norm_data->mutex);
pthread_exit(NULL);
}
int main() {
pthread_t *working_thread;
mat_mult_t *thread_mat_mult_data;
mat_norm_t *thread_mat_norm_data;
pthread_mutex_t *mutex;
double *x, *y, *z, norm;
int i, rows_per_thread;
int n = 8;
int num_of_thrds = 4;// Works when this is 2, not when 4
if(n<=num_of_thrds && num_of_thrds < MAXTHRDS){
printf("Matrix dim must be greater than num of thrds\nand num of thrds less than 124.\n");
return (-1);
}
x = malloc(n*n*sizeof(double));
y = malloc(n*n*sizeof(double));
z = malloc(n*n*sizeof(double));
initMat(n, x);
initMat(n, y);
working_thread = malloc(num_of_thrds * sizeof(pthread_t));
thread_mat_mult_data = malloc(num_of_thrds * sizeof(mat_mult_t));
rows_per_thread = n/num_of_thrds;
for(i=0;i<num_of_thrds;i++){
thread_mat_mult_data[i].x = x + i * rows_per_thread * n;
thread_mat_mult_data[i].y = y;
thread_mat_mult_data[i].z = z + i * rows_per_thread * n;
thread_mat_mult_data[i].n = n;
thread_mat_mult_data[i].sub_n =
(i == num_of_thrds-1) ? n-(num_of_thrds-1)*rows_per_thread : rows_per_thread;
pthread_create(&working_thread[i], NULL, matrix_mult, (void *)&thread_mat_mult_data[i]);
}
for(i=0;i<num_of_thrds;i++){
pthread_join(working_thread[i], NULL);
}
free(working_thread);
working_thread = malloc(num_of_thrds * sizeof(pthread_t));
thread_mat_norm_data = malloc(num_of_thrds * sizeof(mat_norm_t));
mutex = malloc(sizeof(pthread_mutex_t));
for(i=0;i<num_of_thrds;i++){
thread_mat_norm_data[i].z = z + i * rows_per_thread * n;
thread_mat_norm_data[i].n = n;
thread_mat_norm_data[i].global_norm = &norm;
thread_mat_norm_data[i].sub_n =
(i == num_of_thrds-1) ? n-(num_of_thrds-1)*rows_per_thread : rows_per_thread;
thread_mat_norm_data[i].mutex = mutex;
pthread_create(&working_thread[i], NULL, matrix_norm, (void *)&thread_mat_norm_data[i]);
}
//Stuck running here
for(i=0;i<num_of_thrds;i++){
pthread_join(working_thread[i], NULL);
}
printMat(n, z , "z");
printf("\nRow Sum Norm = %f\n", norm);
free(x);
free(y);
free(z);
free(working_thread);
free(thread_mat_mult_data);
free(thread_mat_norm_data);
pthread_mutex_destroy(mutex);
free(mutex);
return(0);
}
I unsure why it works under certain circumstances and not others, any explanation would be great!

Answer 1

Forgot to initialize the mutex with pthread_mutex_init(mutex, NULL); I am still unsure why it would work with out this for two threads but not more than this?