Changing priority of threads

Hi folks,
   I have a small application running, in which I have two real-time
threads. One has a higher priority than the other one. From the high
priority thread I wanna change the priority of the low priority thread
to make it a high priority one, so that the low priority thread will
get the CPU.

  Can anyone please advise me how it could be done? 

What I did:
 
//global definition
pthread_t t1, t2;
struct sched_param p1, p2;

in the init module:
pthread_create(&t1, NULL, f1, 0);
pthread_create(&t2, NULL, f2, 0);

Inside f1(High priority):************************
p1.sched_priority = 5;
pthread_setschedparam(pthread_self,SCHED_FIFO, &p1);

.........
   //change priority of thread t2
    p2.sched_priority = 8;
    pthread_setschedparam(t2, SCHED_FIFO, &p2);

........

Inside f2(low priority):***************************
p2.sched_priority=3;
pthread_setschedparam(pthread_self(), SCHED_FIFO, &p2);
......

I am finding thread exception whenever I am trying to access and set
scheduling information of t2 from t1. Basically what I want are 2
threads( one is higher and one with lower priority) and after some
time, based on a condition I wanna change the low priority thread to
high priority and vice-e-versa. I am new in rtlinux programming.
I couldn't find similar example with rtlinuxfree distribution. I would
highly appreciate any help.

thanks,
santanu
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