I notice two issues here, one big, one smaller:
- This code suggests that you have a misunderstanding of what a barrier does. A barrier never synchronizes across multiple workgroups. It only synchronizes within a workgroup. The CLK_GLOBAL_MEM_FENCE makes it look like it is global synchronization, but it really isn't. That flag just fences all of the current work item's accesses to global memory. So outstanding writes will be globally observable after a barrier with this flag. But it does not change the barrier's synchronization behavior, which is only at the scope of a workgroup. There is no global synchronization in OpenCL, beyond launching another NDRange or Task.
- The first for loop causes multiple work items to overwrite each others' computation. The indexing of pressure_local with iHyd will be done by each work item with the same iHyd. This will produce undefined results.
Hope this helps.