ThreadSanitizer: data race in _struct module initialization with InterpreterPoolExecutor (free-threading build)

[ashm-dev]

Bug report

Bug description:

ThreadSanitizer detects a data race when using InterpreterPoolExecutor with the free-threading build. Multiple threads simultaneously write to the global variable lilendian_table in the _struct module during module initialization.

The race occurs at:

• Thread T1 and T2: Both write to lilendian_table at _structmodule_exec (lines 2745:31 and 2746:33)
• Location: global 'lilendian_table' of size 840 at 0x7ffff4513ca0 in _struct.cpython-315td-x86_64-linux-gnu.so

Both threads are executing through InterpreterPoolExecutor, importing modules in separate subinterpreters, which triggers concurrent initialization of the _struct extension module.

How to Reproduce

Build configuration:

CC=clang CXX=clang++ ./configure --with-thread-sanitizer --with-pydebug --enable-experimental-jit=yes --with-lto=full --with-tail-call-interp --disable-gil
make -j

Run command:

TSAN_OPTIONS=handle_segv=0 ./python -X dev -X showrefcount bug.py

Test script (bug.py):

from concurrent.futures import InterpreterPoolExecutor
from functools import lru_cache

@lru_cache(maxsize=512)
def fib(n):
    """Function that JIT will compile"""
    if n < 2:
        return n
    return fib(n-1) + fib(n-2)

if __name__ == '__main__':
    with InterpreterPoolExecutor() as t:
        for i in range(2):
            t.submit(fib, i)

Note: The race occurs with any loop iteration count (range(1), range(2), etc.)

TSAN Output

log.txt

Environment

• CPython version: main branch
• Operating System: Linux
• Compiler: Clang
• Build flags: --with-thread-sanitizer --with-pydebug --enable-experimental-jit=yes --with-lto=full --with-tail-call-interp --disable-gil

Additional Information

A second similar data race is also reported for another write in _structmodule_exec at line 2746:33 to the same global variable.

Both races involve InterpreterPoolExecutor threads executing module initialization code concurrently in different subinterpreters.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux