I wrote:
> I wonder whether using
>
> __asm__ __volatile__ ("":::"memory");
>
> would be sufficient as it has a way lower overhead than 
> __sync_synchronize().

Namely, something like the attached patch.

Regarding the original patch submission: Is there a reason that you didn't
include the test case of Deepak from https://gcc.gnu.org/ml/fortran/2015-04/msg00062.html
It should work as -fcoarray=lib -lcaf_single "dg-do run" test.

Tobias