PyGILState_Ensure() deadlocks, why?

[Tomas Ukkonen]

Hi

There was a bug in the example code. I fixed it and it STILL deadlocks (my larger software project deadlocks when I call python from C++).

Updated code:

/* * This code deadlocks on python3-dev 3.12.3 (ubuntu 24.04 lts)
 *
 * g++ python_thread_test.cpp `python3-config --cflags --libs --embed`
 * ./a.out
 *
 * uname:
 * Linux softice 6.8.0-36-generic SMP PREEMPT_DYNAMIC x86_64 GNU/Linux
 */

#include <Python.h>
#include <thread>
#include <vector>
#include <iostream>


void thread_function() {
    // Ensure this thread has the GIL
    PyGILState_STATE gstate = PyGILState_Ensure();

    // Execute some Python code
    PyRun_SimpleString("print('Hello from std::thread!')");

    // Release the GIL
    PyGILState_Release(gstate);
}

int main() {
    // Initialize the Python Interpreter
    Py_Initialize();

    // Create a vector of threads
    std::vector<std::thread> threads;

    // Launch threads
    for (int i = 0; i < 5; ++i) {
        threads.push_back(std::thread(thread_function));
    }

    // Join threads
    for (auto& t : threads) {
        t.join();
    }

    // Finalize the Python Interpreter
    Py_Finalize();

    return 0;
}
sunnuntaina 7. heinäkuuta 2024 klo 10:24 ip, Tomas Ukkonen <tomas.ukkonen at protonmail.ch> kirjoitti:

> Hello
> Is this python c api bug? The following C++ code (g++) deadlocks on Ubuntu Linux.
> 

> /* * This code deadlocks on python3-dev 3.12.3 (ubuntu 24.04 lts)
>  *
>  * g++ python_thread_test.cpp `python3-config --cflags --libs --embed`
>  * ./a.out
>  *
>  * uname:
>  * Linux softice 6.8.0-36-generic SMP PREEMPT_DYNAMIC x86_64 GNU/Linux
>  */
> 

> #include <Python.h>
> #include <thread>
> #include <vector>
> #include <iostream>
> 

> void perform_long_operation() {
>     // Simulate a long-running task
>     std::this_thread::sleep_for(std::chrono::seconds(5));
> }
> 

> void thread_function() {
>     // Ensure this thread has the GIL
>     PyGILState_STATE gstate = PyGILState_Ensure();
> 

>     // Execute some Python code
>     PyRun_SimpleString("print('Hello from std::thread!')");
> 

>     // Release the GIL for long operation
>     Py_BEGIN_ALLOW_THREADS
>     perform_long_operation();
>     Py_END_ALLOW_THREADS
> 

>     // Re-acquire the GIL and execute more Python code
>     gstate = PyGILState_Ensure();
>     PyRun_SimpleString("print('Thread operation completed!')");
> 

>     // Release the GIL
>     PyGILState_Release(gstate);
> }
> 

> int main() {
>     // Initialize the Python Interpreter
>     Py_Initialize();
> 

>     // Create a vector of threads
>     std::vector<std::thread> threads;
> 

>     // Launch threads
>     for (int i = 0; i < 5; ++i) {
>         threads.push_back(std::thread(thread_function));
>     }
> 

>     // Join threads
>     for (auto& t : threads) {
>         t.join();
>     }
> 

>     // Finalize the Python Interpreter
>     Py_Finalize();
> 

>     return 0;
> }
> 

> 

> 

> Tomas Ukkonen
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