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openblas_set_threads_callback_function is a silent no-op on Windows #6108
Description
Summary
On Windows, openblas_set_threads_callback_function() is accepted and silently ignored: the registered callback is never invoked, and no error or warning is produced. The symbol is exported and declared in cblas.h, so callers have no way to tell that caller-managed threading is not in effect.
This looks like fallout from #4835 rather than an intentional limitation.
History
The win32 backend did have the hook:
- 7102367 ("Introduced callback to Pthread, Win32 and OpenMP backend") added it, on top of the file as it stood after Improve Windows threading performance scaling #4359.
- Temporarily revert to the coarse-grained locking in the Windows thread server #4835 undid Improve Windows threading performance scaling #4359 by replacing driver/others/blas_server_win32.c with its pre-Improve Windows threading performance scaling #4359 revision.
Question
Is this worth restoring, or is caller-managed threading deliberately unsupported on Windows?
Reacted by uuiitwp
Activity
martin-frbg commentedon Oct 8, 2026
Ah yes that looks as if someone (i.e. me) did not realize why git refused to revert the commit. I'll see that I restore the callback tomorrow.
uuiitwp commentedon Oct 9, 2026
Thanks!
uuiitwp commentedon Oct 26, 2026
Hi @martin-frbg , just a friendly ping that this branch currently has merge conflicts and cannot be merged. Could you please resolve them when you have time? Thanks!
martin-frbg commentedon Oct 26, 2026
Yes I'm aware - but more importantly, that PR led to a hang on Windows so is not the correct fix. I'll need more time and quiet for this than for the mostly mechanical import of Reference-LAPACK updates that I'm currently trying to fit in. It doesn't really help that both projects are seeing a wave of AI-generated PRs now.