The rdseed instruction with multiple threads

Question

I would like to generate seeds for a custom PRNG using the rdseed instruction with multiple cores.

Here is what I have so far using OpenMP.

//gcc -Wall -O3 -fopenmp -mrdseed myrand.c
#include <x86intrin.h>
#include <stdio.h>
int main(void)  {  
    #pragma omp parallel
    {
        unsigned r;
        #pragma omp critical
        while(!_rdseed32_step(&r));
        //prng_init(r);
        printf("%d\n", r);
    }
}

Is this the correct/ideal way to generate a seed for each thread? Do I need the critical section when I call rdseed. The _rdseed32_step intrinsic returns 1 if a random value was generated and 0 otherwise.

The Intel DRNP Manual

4.3.1 Retry Recommendations

Unlike the RDRAND instruction, the seed values come directly from the entropy conditioner, and it is possible for callers to invoke RDSEED faster than those values are generated. This means that applications must be designed robustly and be prepared for calls to RDSEED to fail because seeds are not available (CF=0).

If only one thread is calling RDSEED infrequently, it is very unlikely that a random seed will not be available. Only during periods of heavy demand, such as when one thread is calling RDSEED in rapid succession or multiple threads are calling RDSEED simultaneously, are underflows likely to occur. Because the RDSEED instruction does not have a fairness mechanism built into it, however, there are no guarantees as to how often a thread should retry the instruction, or how many retries might be needed, in order to obtain a random seed. In practice, this depends on the number of hardware threads on the CPU and how aggressively they are calling RDSEED.

As far as I understand there is only one seed generator per processor so seeds can't be generated in parallel and since it takes time to generate a seed the correct solution to me seems to be to have each core/hyper-thread ask for a thread one at a time and have the thread that is calling rdseed wait until it gets a seed.

Since I only need one seed per thread then

#pragma omp critical
while(!_rdseed32_step(&r));

seems to be the right approach to me.

Answer 1

It will work. In the very unlikely event that the RNG broke in the middle of running this code, it would lock up in a tight loop, but in realistic scenarios, it will get you a random number and all will be fine.

_rdseed16_step(), _rdseed32_step() and _rdseed64_step() follow the semantics of the RdSeed instruction, which is to return a success bit in CF and the random number in the target register. So it can be run in a loop to get as many random bits are required.