Fortran Modules with OpenMP - fortran

Question

It seems to me that fortran modules, which can be used to hold global variables across subroutines, don't work the same when using OpenMP. Here's an example:
main.f90
program main
use mod
implicit none
!$OMP PARALLEL private(a)
!$OMP DO
do i=1,10
a=i-1
print*,"a =",a
call echo
print*,"b =",b
enddo
!$OMP END DO
!$OMP END PARALLEL
end program main
echo.f90
subroutine echo
use mod
implicit none
b=a+1
!print *,a,"+1=",b
end subroutine echo
mod.f90
module mod
integer:: i,a,b
end module mod
Now if you compile and run this without OpenMP you get:
a = 0
b = 1
a = 1
b = 2
a = 2
.....ect. This is what you'd expect
But, if you compile WITH openMP you get:
a = 7
b = 1
a = 6
b = 1
a = 8
.....ect. This is not what I want. I know that the echo subroutine is getting 'a' from the module, not the private 'a' that the thread has. Is there any way to do this besides passing it as an argument? There are a ton of variables in my module and it would be tedious.

Answer 1

Inside the procedure echo, a and b are variables that are referenced in a region but not in a construct - execution wise they appear in between a matching !$OMP PARALLEL and !$OMP END PARALLEL directive, but source wise they do not. As they are module variables, and in the absence of directives to the contrary, the rules for data sharing attributes in 2.14.1.2 of the OpemMP 4.0 standard specify that those variables inside the procedure are shared.
Consequently your example code has a data race, with multiple threads writing to b inside the echo subroutine without synchronization.
You can use the THREADPRIVATE directive in the module to change the data sharing attribute of those module variables. You will need to remove the private specification for a at the same time.
In the long run, a far better approach may be to make the flows of information in your program explicit to a reader of the code (and more flexibly configurable by a code writer), by passing information as arguments (perhaps bundled together in derived types) rather than hiding those flows through the use of global (module) variables.