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From: "tkoenig at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug fortran/97589] Segementation fault when allocating coarrays.
Date: Thu, 26 Nov 2020 22:41:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97589

--- Comment #22 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
Hi Toon,

it took some time, but we finally figured out that it is actually
a bug in your program that is causing problems.

It has (shortened)

nxglobal =3D 72;

This sets the coarray nxglobal to 72 on every image, including image 2.

if (this_image() =3D=3D 1) then
   read(*,config)

This optionally reads in nxglobal[1].

!     Why won't this work as nxglobal[:] =3D nx ?
   do i =3D 2, num_images()
      nxglobal[i] =3D nxglobal;

This sets nxglobal[2] on image 2 from image 1. This is a race
condition: It is not clear which store comes first.

To fix this, you would need a "sync all" before the
if (this_image() =3D 1) statement, or you could set the default
value on image 1 only.

(Incidentally, looking at this code led to finding a bug in
namelist handling, where the implied this_image() was not honored
(namelist I/O only worked on image 1), so the time looking at this
bug was not wasted :-)

By the way, there is also a segfault with GFORTRAN_NUM_IMAGES=3D64, which
will need to be investigated.=