cuda unified memory and pointer aliasing - cuda

Question

I am refreshing my mind with cuda, specially the unify memory (my last real cuda dev was 3 years ago), I am a bit rusted.
The pb:
I am creating a task from a container using unify memory. However, I get a crash, after a few days of investigation,
I am not able to say where is the crash (copy constructor), but not why. Because all pointers are allocated correctly.
I am not in contraction with Nvidia post (https://devblogs.nvidia.com/parallelforall/unified-memory-in-cuda-6/)
about C++ and unify memory
#include <cuda.h>
#include <cstdio>
template<class T>
struct container{
container(int size = 1){ cudaMallocManaged(&p,size*sizeof(T));}
~container(){cudaFree(p);}
__device__ __host__ T& operator[](int i){ return p[i];}
T * p;
};
struct task{
int* a;
};
__global__ void kernel_gpu(task& t, container<task>& v){
printf(" gpu value task %i, should be 2 \n", *(t.a)); // this work
task tmp(v[0]); // BUG
printf(" gpu value task from vector %i, should be 1 \n", *(tmp.a));
}
void kernel_cpu(task& t, container<task>& v){
printf(" cpu value task %i, should be 2 \n", *(t.a)); // this work
task tmp(v[0]);
printf(" cpu value task from vector %i, should be 1 \n", *(tmp.a));
}
int main(int argc, const char * argv[]) {
int* p1;
int* p2;
cudaMallocManaged(&p1,sizeof(int));
cudaMallocManaged(&p2,sizeof(int));
*p1 = 1;
*p2 = 2;
task t1,t2;
t1.a=p1;
t2.a=p2;
container<task> c(2);
c[0] = t1;
c[1] = t2;
//gpu does not work
kernel_gpu<<<1,1>>>(c[1],c);
cudaDeviceSynchronize();
//cpu should work, no concurent access
kernel_cpu(c[1],c);
printf("job done !\n");
cudaFree(p1);
cudaFree(p2);
return 0;
}
Objectively I can pass an object as an argument where the memory has been allocated properly. However, it look like it not possible to use a second degree
of indirection (here the container)
I am doing a conceptual mistake, but I do not see where.
Best,
Timocafe
my machine: cuda 7.5, gcc 4.8.2, Tesla K20 m

Answer 1

Although the memories were allocated as Unified Memory, the container itself is declared in host code and allocated in host memory: container<task> c(2);. You can not pass it as a reference to the device code, and de-referencing it in a kernel will very likely result in illegal memory access.
You may want to use cuda-memcheck to identify such issues.