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).