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I'm trying to a write a simple compute shader which will take in an array of points as well as two points which define a line. I would like to invoke the shader for each point in the array, have all the distances calculated in parallel and then have each invocation wait it's turn in a busy loop to update the farthest point data. My compute shader looks like this

#version 430 core

layout(std430, binding = 0) buffer BufferObjectIn {
    vec3 data[];

} pointData;

layout(std430, binding = 1) buffer BufferObjectOut{
    uint inLeftIndex;
    uint inRightIndex;
    uint inPointCount;
    int outFarthestIndex; 
    float outFarthestDist;
    int mutex;
} ssbo;

layout(local_size_x = 64) in;

float signedDistance(vec3 linePoint1, vec3 linePoint2, vec3 p) {
   return (linePoint2.x - linePoint1.x) * (linePoint1.y - p.y) - (linePoint2.y - linePoint1.y) * (linePoint1.x - p.x);
}

void main(void) {
    uint threadIndex = gl_GlobalInvocationID.x;

    if (threadIndex >= ssbo.inPointCount) {
        return;
    }

    vec3 leftPoint = pointData.data[ssbo.inLeftIndex];
    vec3 rightPoint = pointData.data[ssbo.inRightIndex];
    vec3 thisPoint = pointData.data[threadIndex];

    float dist = signedDistance(leftPoint, rightPoint, thisPoint);

    if (dist < 0){
        return;
    }

    do {
        lock = atomicCompSwap(ssbo.mutex, 0, 1);
    } while (lock == 0);

    barrier();
    if (dist > ssbo.outFarthestDist) {
        ssbo.outFarthestDist = dist;
        ssbo.outFarthestIndex = int(threadIndex);
    }

    ssbo.mutex = 0;
}

What I'm finding is that my application will show different results even though I am running it on the same set of data. Have I done something wrong?

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