using spinlock to control image read/writes
I'm working on a fragment shader to read and write to image arrays. So that only one shader invocation can access a given element in the image at once, I've implemented a spinlock image to control access to the images. The problem is that the spinlock keeps running into deadlock conditions. Here's my fragment shader code:
#version 420 core
coherent uniform layout(rgba8) image2DArray ColorImages;
// 0 = element available for read/writes; 1 = element is locked by another invocation
coherent uniform layout(r32ui) uimage2D SpinlockImage;
const ivec2 coords = ivec2(gl_FragCoord.xy);
// obtain the lock
while (imageAtomicCompSwap(SpinlockImage, coords, 0, 1) != 0);
// do something with ColorImages. As a simple example, load value from layer 0 and write to layer 1
vec4 val = imageLoad(ColorImages, ivec3(coords, 0));
imageStore(ColorImages, ivec3(coords, 1), val);
// ensure that image writes are visible to other shader invocations
// unlock the spinlock
imageAtomicExchange(SpinlockImage, coords, 0);
My understanding of imageAtomicCompSwap() in the above context is that only one shader invocation will read a value of zero from the image. In the same atomic operation, the image value will be set to 1. This in effect will lock out all other invocations, causing them to spin in the while loop until the invocation that has the lock sets the image value back to 0.
Why would this code cause a deadlock?
When the deadlock happens on my Windows 7 system, I get this message in a window:
"The NVIDIA OpenGL driver lost connection with the display driver due to exceeding the Windows Time-Out limit and is unable to continue.
The application must close."
I did a little research and the Windows Time-Out limit is two seconds.