SCHED_FIFO priority respect

It seems that the I do not manage to change correctly the priority of my threads with scheduling policy SCHED_FIFO.

The main thread with fifo priority 10 will launch the thread divisor and ratio.

divisor thread should get priority 2 so that the ratio thread with priority 1 will not evaluate a/b before b gets a decent value ( this is a completely hypothetical scenario only for the MCVE, not a real life case with semaphores or condition variables ).

Any hint welcome !

Note: environnment under Virtualbox

Normal execution OK

johndoe@VirtualBox:~/Code/gdb_sched_fifo$ g++ main.cc -o main -pthread
johndoe@VirtualBox:~/Code/gdb_sched_fifo$ ./main
Result: 0.333333

Normal execution KO

johndoe@VirtualBox:~/Code/gdb_sched_fifo$ g++ main.cc -o main -pthread
johndoe@VirtualBox:~/Code/gdb_sched_fifo$ ./main
Result: Inf

The MCVE:

#include <iostream>
#include <thread>

double a = 1.0F;
double b = 0.0F;

void ratio(void)
{
    struct sched_param param;
    param.sched_priority = 1;
    pthread_setschedparam(pthread_self(),SCHED_FIFO,&param);
    //error checking not shown for MCVE

    std::cout << "Result: " << a/b << "\n" << std::flush;
}

void divisor(void)
{
    struct sched_param param;
    param.sched_priority = 2;
    pthread_setschedparam(pthread_self(),SCHED_FIFO,&param);

    b = 3.0F;

    std::this_thread::sleep_for(std::chrono::milliseconds(2000u));
}


int main(int agrc, char * argv[])
{
    struct sched_param param;
    param.sched_priority = 10;
    pthread_setschedparam(pthread_self(),SCHED_FIFO,&param);

    std::thread thr_ratio(ratio);
    std::thread thr_divisor(divisor);

    thr_ratio.join();
    thr_divisor.join();

    return 0;
}