[Bug 106902] Certain uses of atomic operations causes infinite loop due to helper invocations

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106902

            Bug ID: 106902
           Summary: Certain uses of atomic operations causes infinite loop
                    due to helper invocations
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: caio.oliveira at intel.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Certain uses of atomic operations, e.g. open coding atomicAdd by using a loop
with loads and atomicCompSwaps, will not work when executed in helper
invocations, leading to infinite loops.

    layout(binding = 0) buffer bufblock {
        int value;
    };

...

        int f;

        /* This is an open-coded atomicAdd. */
        do {
            f = value;
        } while (f != atomicCompSwap(value, f, f + 4));

This code can lead to hangs, since a helper invocation will loop indefinetely,
because atomicCompSwap don't write anything and might return junk while the "f
= value" reads an real value.

"Atomic operations to image, buffer, or atomic counter variables performed by
helper invocations have no effect on the underlying image or buffer memory. The
values returned by such atomic operations are undefined."

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/gl_HelperInvocation.xhtml

Full shader codeis available in attachment of bug
https://bugs.freedesktop.org/show_bug.cgi?id=106774.

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