Merely linking to OpenBLAS with MinGW 32-bit causes hangs

[Szabolcs Horvát]

Hello,

We have seen random hangs when using OpenBLAS from MSYS2 / mingw-w64-i686 (i.e. 32-bit), and I was wondering if anyone has bumped into the same problem and has can give any hints about what may be going wrong.

More specifically:

We see regular hangs when running igraph's test suite with 32-bit MinGW on GitHub Actions. Unfortunately, I cannot reproduce it on a local machine. Here is one example:

https://github.com/igraph/igraph/runs/2365429927#step:6:780

The hang happens only when igraph is linked to OpenBLAS (either directly, or indirectly through MSYS2's ARPACK). It does not happen when using the reference BLAS. The test programs that hang are different every time and do not usually call any BLAS/LAPACK functions at all. They simply happen to load the OpenBLAS DLL. Strangely, it is usually only one test program that hangs (out of 380+) during each run of the test suite.

I have seen the FAQ item about multi-threaded programs, but igraph itself is not multi-threaded (OpenMP was explicitly disabled). https://github.com/xianyi/OpenBLAS/wiki/faq#multi-threaded

Has anyone else seen anything similar?

[Szabolcs Horvát]

I wanted to note that the hang does not happen with OPENBLAS_NUM_THREADS=1, but it does with OPENBLAS_NUM_THREADS=2 (or without specifying OPENBLAS_NUM_THREADS).