Fix race condition in OpenCL kernel #3535
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Without the barrier at the end of barrierOR, it is possible for work-item 0 to start the next loop iteration and update predicates[0] while other work-items are still inside barrierOR reading `predicates`, meaning they read the next loop iteration's exit condition. This results in a divergent loop, where not all work-items reach the same barriers. A previous fix identified this as a problem only on NVIDIA platforms, but strictly speaking a barrier is required in all cases to avoid a spec violation and undefined behaviour.
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Took me a bit to figure out the problem but I see the issue now. The we can ignore the errors in the CI because they are not related. I will test it on a couple of other systems before merge this PR. Thank you for your contribution! |
christophe-murphy
approved these changes
Feb 20, 2025
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Description
Without the barrier at the end of barrierOR, it is possible for work-item 0 to start the next loop iteration and update predicates[0] while other work-items are still inside barrierOR reading
predicates, meaning they read the next loop iteration's exit condition. This results in a divergent loop, where not all work-items reach the same barriers.A previous fix identified this as a problem only on NVIDIA platforms, but strictly speaking a barrier is required in all cases to avoid a spec violation and undefined behaviour.
Changes to Users
The kernel should produce correct results on more OpenCL implementations.
Locally I tested both
Intel(R) FPGA Emulation Deviceand various oneAPI Construction Kit devices, which all previously failed theconfidence_connected_opencl --gtest_filter="SingleSeed/ConfidenceConnectedDataTest.SegmentARegion/_prefix_background_radius_0_multiplier_1_iterations_5_replace_255"unit test.I'm unable to test other OpenCL implementations, sorry.
Checklist
[ ] Functions added to unified API[ ] Functions documented