[Bug fortran/99529] New: libgfortran I/O: Data races related to new unit / new unit calls for I/O to strings

[burnus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99529

            Bug ID: 99529
           Summary: libgfortran I/O: Data races related to new unit / new
                    unit calls for I/O to strings
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: jvdelisle at gcc dot gnu.org
  Target Milestone: ---

Reported at https://gcc.gnu.org/pipermail/fortran/2021-March/055795.html

Using helgrind on a simple omp do loop with write to
a character variable, I get some possible data races
in libgfortran/io/unit.c.

There global array newunits is
allocated and possibly reallocated in
"newunit_alloc". According to the lock outputs from
helgrind I see that this routine is called even if
output is into a character variable.

Routine "newunit_alloc" uses a lock to avoid having several
threads all over the place. But newunit_free also
writes to newunits array. And this routine does not
obtain a lock itself (see comment above newunit_free
in unit.c) So in theory it can happen that newunit_alloc
reallocated newunits, and newunit_free writes to it just
at this time. As I also use 18 threads the initial size of 16
does not suffice and reallocation does probably
indeed happen.

Also access to newunit_lwi is not protected as well
(and complained about by helgrind).