[Openmp-commits] [PATCH] D104418: [PoC][WIP][OpenMP][Offloading] Fixed data race in libomptarget caused by async data movement

Thu Jun 17 09:49:27 PDT 2021

ye-luo added a comment.

In D104418#2824984 <https://reviews.llvm.org/D104418#2824984>, @JonChesterfield wrote:

>> Since the mapping is already there, T2 will NOT issue data transfer. This can improve performance. It doesn't make sense to transfer same data for several times.
>
> That doesn't sound right. There's no reason to expect the data that was copied across will not be modified by the first region. The openmp semantics look like copy to device at target region start and copy back at the end. Picking up partially written data from another executing kernel breaks that.
>
> Given two target regions that both map the same data, I'm fairly sure both maps have to happen, unless we do significantly more thorough analysis to determine that the output of one kernel is definitely the same as the input to the next, in which case we would be safer fusing them.

I don't agree with you. T2 should follow the reference counting transfer behavior and not issue data transfer. If T1 change things on the device and surprise T2, it is a bug in the user code, user is responsible to express dependency on the target tasks.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104418/new/

https://reviews.llvm.org/D104418