# Avoiding Race Condition

Hi experts,

I am a beginner in CUDA programming. I am trying to parallelize a serial code which finds a convex envelop from a set of given points in a 2D plane.

The serial code looks like this :

void find_edges_on_host(float * h_x, float * h_y, int *h_edges)
{
//loop on all possible edges == pairs of points
for (int i=0; i<NPOINTS; i++)
{
for (int j=0; j<NPOINTS; j++)
{
if (i>=j)
{
continue; //edge i,j == edge j,i
}

``````		//all the others points should be on the same side of the edge i,j

//normal to the edge (unnormalized)
float nx= - ( h_y[j]- h_y[i]);
float ny= h_x[j]- h_x[i];

.........
.........
// This is where race condition occurs
if (isValid)
{
//write the edge to h_edges in the first available position
if (h_edges[2*i]==-1)
{
h_edges[2*i]=j;
}
else
{
h_edges[2*i+1]=j;
}

//we write the edge two times for a direct access in the next stage
if (h_edges[2*j]==-1)
{
h_edges[2*j]=i;
}
else
{
h_edges[2*j+1]=i;
}

}
}
}
return;
``````

}

For paralleizing, the for loops can be removed by using 2D threads. My problem lies with the ‘if’ block. For example, edge 1-2 and edge 1-3 are valid, then we have 2 threads with the same threadIdx.x = 1 who are trying to execute the statement

h_edges[2*i]=j;

Similarly there are other race condition possibilities exist.

My question is, is there any way I can remove this race condition without making use of functions like ‘AtomicAdd’ etc. ?
Can I re-structure the ‘if’ block in such a way that race condition will not occur ??

Kind regards