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GeezerVison is showing github.com — gh-116738: Make cmath module thread-safe by yoney · Pull Request #142161 · python/cpython

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@yoney

@yoney yoney commented Dec 1, 2025 •
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While investigating thread safety in the cmath module, I found that the module is generally thread-safe. However, there is a potential race condition during the initialization of the special value tables in cmath_exec(). This is usually safe, but with subinterpreters, a race condition could occur.

The fix converts all special value tables from runtime initialization to static initialization, eliminating the race condition.

I have also created a test to run under ThreadSanitizer. However, I am not including the test in the current PR because, although this PR addresses the issue in the cmath module, ThreadSanitizer still reports other issues below when subinterpreters and free-threading are used together. I plan to address those issues.

...
SUMMARY: ThreadSanitizer: data race /workspace/cmath_tsan/./Modules/posixmodule.c:18691 in posixmodule_exec
...
SUMMARY: ThreadSanitizer: data race /workspace/cmath_tsan/Objects/exceptions.c:4547 in _PyBuiltins_AddExceptions
...
SUMMARY: ThreadSanitizer: data race /workspace/cmath_tsan/Python/crossinterp_exceptions.h:154 in init_static_exctypes
...
...

cc: @mpage @colesbury

  • Issue: Audit all built-in modules for thread safety #116738

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@yoney
pythongh-116738: Make cmath module thread-safe
8b961ad
@bedevere-app bedevere-app Bot mentioned this pull request Dec 1, 2025
Audit all built-in modules for thread safety #116738
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colesbury
colesbury reviewed Dec 1, 2025
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Generally looks good. It's subinterpreter specific and not really free threading related (i.e., you'll have the same data races with the default GIL enabled build).

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Comment thread Modules/cmathmodule.c Resolved

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Comment thread Tools/c-analyzer/cpython/ignored.tsv Resolved

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Comment thread Misc/NEWS.d/next/Core_and_Builtins/2025-12-01-10-03-08.gh-issue-116738.972YsG.rst Resolved

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@yoney
yoney marked this pull request as ready for review December 1, 2025 20:58
@yoney
yoney requested a review from ericsnowcurrently as a code owner December 1, 2025 20:58
@bedevere-app bedevere-app Bot added the awaiting review label Dec 1, 2025
@yoney
Address review comments
90be0d7
@yoney

yoney commented Dec 1, 2025

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Thank you for the review, Sam!

It's subinterpreter specific and not really free threading related.

Yes, I agree, this is related to subinterpreters. I was mainly checking the module for ft-python, which is why I created the PR under gh-116738 to mark it checked.
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skirpichev
skirpichev reviewed Dec 2, 2025
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Comment thread Modules/cmathmodule.c Resolved

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yoney added 3 commits December 2, 2025 11:17
@yoney
Static initialize trigonometric tables
90ab34c
@yoney
Address Windows compiler error
a365734
@yoney
Remove C(R, I) macro
64254ba
skirpichev
skirpichev reviewed Dec 3, 2025
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Looks good, but could you double check and verify that all functions include tests for all combinations of special values? I.e. when one component is a zero, infinity or nan, same other (or is a finite nonzero number, if the first is nonzero).

Tests are here: Lib/test/mathdata/cmath_testcases.txt (usually in a dedicated section, like "special values").

Test script 
#!/bin/sh

# a.sh

file=Lib/test/mathdata/cmath_testcases.txt
funcs="acos acosh asinh atanh cosh exp log sinh sqrt tanh rect"
zeros="0.0 -0.0"
nonfinite="-inf nan inf"
all="$zeros $nonfinite [1-9]\.[0-9].* -[1-9]\.[0-9].*"
for f in $funcs
do
  for t1 in $zeros
  do
    for t2 in $zeros $nonfinite
    do
      re=" $f $t1 $t2 -> "
      grep -q "$re" $file || echo "$f $t1 $t2 - missing"
      re=" $f $t2 $t1 -> "
      grep -q "$re" $file || echo "$f $t2 $t1 - missing"
    done
  done
  for t1 in $nonfinite
  do
    for t2 in $all
    do
      re=" $f $t1 $t2 -> "
      grep -q "$re" $file || echo "$f $t1 $t2 - missing!"
      re=" $f $t2 $t1 -> "
      grep -q "$re" $file || echo "$f $t2 $t1 - missing!"
    done
  done
done

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@skirpichev
skirpichev self-requested a review December 3, 2025 00:20
skirpichev
skirpichev approved these changes Dec 3, 2025
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LGTM

I've checked that special values are tested, though appreciate a second look. Please adjust PR description (an maybe title).

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@bedevere-app bedevere-app Bot added awaiting core review and removed awaiting review labels Dec 3, 2025
@yoney

yoney commented Dec 3, 2025

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I've checked that special values are tested, though appreciate a second look. Please adjust PR description (an maybe title).

Thank you so much, @skirpichev

I’ve checked them as well and they look good to me. I am just sharing how I verified this locally, since it’s easy to accidentally copy-paste something incorrectly and not notice it: I compared the statically initialized version with the previous version before deleting the old tables from the code.
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colesbury
colesbury approved these changes Dec 3, 2025
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LGTM

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels Dec 3, 2025
@colesbury colesbury added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Dec 3, 2025
@bedevere-bot

bedevere-bot commented Dec 3, 2025

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🤖 New build scheduled with the buildbot fleet by @colesbury for commit 64254ba 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F142161%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.
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@bedevere-bot bedevere-bot removed the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Dec 3, 2025
@colesbury

colesbury commented Dec 4, 2025

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Buildbot failures are unrelated
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@colesbury
colesbury merged commit 2dac9e6 into python:main Dec 4, 2025
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@bedevere-app bedevere-app Bot removed the awaiting merge label Dec 4, 2025
@colesbury colesbury added the needs backport to 3.14 bugs and security fixes label Dec 4, 2025
@miss-islington-app

miss-islington-app Bot commented Dec 4, 2025

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Thanks @yoney for the PR, and @colesbury for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖
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miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Dec 4, 2025
@yoney @miss-islington
pythongh-116738: Statically initialize special constants in cmath mod…
903a785 
…ule (pythongh-142161)

The initialization during `mod_exec` wasn't thread-safe with multiple interpreters.
(cherry picked from commit 2dac9e6)

Co-authored-by: Alper <[email protected]>
@bedevere-app

bedevere-app Bot commented Dec 4, 2025

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GH-142261 is a backport of this pull request to the 3.14 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Dec 4, 2025
colesbury pushed a commit that referenced this pull request Dec 4, 2025
@miss-islington @yoney
[3.14] gh-116738: Statically initialize special constants in cmath mo…
94012d6 
…dule (gh-142161) (gh-142261)

The initialization during `mod_exec` wasn't thread-safe with multiple interpreters.
(cherry picked from commit 2dac9e6)

Co-authored-by: Alper <[email protected]>
StanFromIreland pushed a commit to StanFromIreland/cpython that referenced this pull request Dec 6, 2025
@yoney @StanFromIreland
pythongh-116738: Statically initialize special constants in cmath mod…
0f1fae1 
…ule (pythongh-142161)

The initialization during `mod_exec` wasn't thread-safe with multiple interpreters.
@yoney
yoney deleted the ft_cmath branch January 27, 2026 17:07
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@ZeroIntensity ZeroIntensity ZeroIntensity left review comments
@colesbury colesbury colesbury approved these changes
@skirpichev skirpichev skirpichev approved these changes
@ericsnowcurrently ericsnowcurrently Awaiting requested review from ericsnowcurrently

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@yoney @bedevere-bot @colesbury @skirpichev @ZeroIntensity
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