[Openmp-commits] [PATCH] D145831: [OpenMP][libomptarget] Add support for critical regions in AMD GPU device offloading

[Gheorghe-Teodor Bercea via Phabricator via Openmp-commits]

doru1004 added a comment.

In D145831#4207338 <https://reviews.llvm.org/D145831#4207338>, @JonChesterfield wrote:

> I'm really sure that locks at thread scope do not work on amdgpu or pre-volta nvptx. One of the threads wins the cas, all the others do not, and it immediately deadlocks.
>
> Critical sections can be done by rewriting the cfg, general purpose locks can't.
>
> What am I missing here?

What's missing is the outer loop which only allows the lowest thread in the wave to actually be active. All the other inactive wave threads are waiting at a synchronization point. Once the first thread in the wave executes the critical it joins the other threads and loops around again but this time the next thread will be active and so on. Does this help answer your question? Some of that code requires inspection of LLVM IR if you want to see it. This here is just the runtime bit.


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  https://reviews.llvm.org/D145831/new/

https://reviews.llvm.org/D145831