[BUG] Missing threadfence in grid sync

[AKKamath]

In /cpp/src_prims/common/grid_sync.cuh only the masterThread() executes a threadfence (line 191).
As per CUDA documentation, threadfence semantics are only guaranteed for the calling thread.
For other threads, there is no guarantee that global writes are visible to all threads.
The threadfence at line 192 should be moved out of the if condition to line 189.

An example can be seen in CUB's equivalent implementation of a grid barrier.
https://github.com/NVIDIA/cub/blob/main/cub/grid/grid_barrier.cuh
You'll notice a threadfence at line 78 executed by all threads participating in the barrier.
The comment at line 76 also confirms that this fence is to ensure the visibility of global writes.

[divyegala]

@AKKamath are you using this prim somewhere? I think we should replace this prim with libcu++'s synchronization primitives

[AKKamath]

I was trying out this primitive for synchronizing different threadblocks.
When going through its implementation, I noticed this issue.
Can you confirm that this is a bug and should I use a different implementation of grid sync?

[divyegala]

Yeah, I'm it's probably a bug. When used somewhere in a kernel, you definitely want all threads to wait for their writes to be visible. Why not just use CUB's implementation directly, and we can remove the one from cuML?

[AKKamath]

Alright. Thanks for the reply. I'll use CUB instead.

[dantegd]

@divyegala do you know how much the primitive in src_prims is used around the codebase?

[divyegala]

@dantegd I don't see it anywhere

[github-actions]

This issue has been labeled inactive-30d due to no recent activity in the past 30 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed. This issue will be labeled inactive-90d if there is no activity in the next 60 days.

[github-actions]

This issue has been labeled inactive-90d due to no recent activity in the past 90 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed.