Tutorial: Introduction to CUDA Fortran | GTC 2013

Computing π with an Atomic Lock
Instead of storing back the partial sum:
! Each block writes back its partial sum
if (index == 1) partial(BlockIdx%x)=psum(1)
use an atomic lock to ensure that one block at the time updates the final sum:
if (index == 1) then
do while ( atomiccas(lock,0,1) == 1) !set lock
end do
partial(1)=partial(1)+psum(1) ! atomic update of partial(1)
call threadfence() ! Wait for memory transaction to be visible to all the other threads
lock =0 ! release lock
end if
partial(1)=0
call sum(deviceData,partial,N)
inside=partial(1)