GLSL SpinLock only Mostly Works

Question

I have implemented a depth peeling algorithm using a GLSL spinlock (inspired by this). In the following visualization, notice how overall the depth peeling algorithm functi

Answer 1

For reference, here is locking code that has been tested to work on Nvidia driver 314.22 & 320.18 on a GTX670. Note that existing compiler optimization bugs are triggered if the code is reordered or rewritten to logically equivalent code (see comments below.) Note in the below I use bindless image references.

// sem is initialized to zero
coherent uniform layout(size1x32) uimage2D sem;

void main(void)
{
    ivec2 coord = ivec2(gl_FragCoord.xy);

    bool done = false;
    uint locked = 0;
    while(!done)
    {
     // locked = imageAtomicCompSwap(sem, coord, 0u, 1u); will NOT work
        locked = imageAtomicExchange(sem, coord, 1u);
        if (locked == 0)
        {
            performYourCriticalSection();

            memoryBarrier();

            imageAtomicExchange(sem, coord, 0u);

            // replacing this with a break will NOT work
            done = true;
        }
    }

    discard;
}

Answer 2

The "imageAtomicExchange(img2D_0,coord,0);" needs to be inside the if statement, since it is resetting the lock variable even for threads that didn't have it! Changing this fixes it.