[rogerbinns]

C code needs to be updated to be safe in a GIL free execution environment. It is a lot of work! The pervasive problem is that mutable data structures (lists, dict etc) could change at any arbitrary point while the C code is working with them, and the reference count for others could drop to zero if *anyone* is using a borrowed reference (common for performance in CPython APIs). Previously the GIL protected where those changes could happen. In simple cases it is adding a critical section, but often there multiple data structures in play. As an example these are the changes that had to be done to the standard library json module:

https://github.com/python/cpython/pull/119438/files#diff-efe...

This is how much of the standard library has been audited:

https://github.com/python/cpython/issues/116738

The json changes above are in Python 3.15, not the just released 3.14.

The consequences of the C changes not being made are crashes and corruption if unexpected mutation or object freeing happens. Web services are exposed to adversity so be *very* careful.

It would be a big help if CPython released a tool that could at least scan a C code base to detect free threaded issues, and ideally verify it is correct.

[dehrmann]

I think Java got this mostly right. On the threading front, very little is thread-safe or atomic (x += 1 is not thread-safe), so as soon as you expose something to threads, you have to think about safe access. For interacting with C code, your choices are either shared buffers or copying data between C and Java. It's painful, but it's needed for memory safety.

[rogerbinns]

The core Python data structures are atomic to Python developers. eg there is no way you can corrupt a list or dictionary no matter how much concurrency you try to use. This was traditionally done under the protection of the global interpreter lock which ensured that only one piece of C code at a time was operating with the internals of those objects. C code can also release the GIL eg during I/O, or operations in other libraries that aren't interacting with Python objects, allowing concurrency.

The free threaded implementation adds what amounts to individual object locks at the C level (critical sections). This still means developers writing Python code can do whatever they want, and they will not experience corruption or crashes. The base objects have all been updated.

Python is popular because of many extensions written in C, including many in the standard library. Every single piece of that code must be updated to operate correctly in free threaded mode. That is a lot of work and is still in progress in the standard library. But in order to make the free threaded interpreter useful at this point, some have been marked as free thread safe, when that is not the case.

[colonCapitalDee]

Is compound assignment atomic in any major language?

[hunterpayne]

> x += 1 is not thread-safe

Nit, that's true iff x is a primitive without the volatile modifier. That's not true for a volatile primitive.

[westurner]

> It would be a big help if CPython released a tool that could at least scan a C code base to detect free threaded issues, and ideally verify it is correct.

Create or extend a list of answers to:

What heuristics predict that code will fail in CPython's nogil "free threaded" mode?

[rogerbinns]

Some of that is already around, but scattered across multiple locations. For example there is a list in the Python doc:

https://docs.python.org/3/howto/free-threading-extensions.ht...

And a dedicated web site:

https://py-free-threading.github.io/

But as an example neither include PySequence_Fast which is in the json.c changes I pointed to. The folks doing the auditing of stdlib do have an idea of what they are looking for, and so would be best suited to keep a list (and tool) up to date with what is needed.

[sgammon]

> “at least scan a C code base to detect free threaded issues”

if such a thing were possible, thread coordination would not have those issues in the first place

[rogerbinns]

Some examples of what it could do when using the C Python APIs:

* Point out using APIs that return borrowed references

* Suggest assertions that critical sections are held when operating on objects

* Suggest alternate APIs

* Recognise code patterns that are similar to those done during the stdlib auditing work

The compiler thread sanitizers didn't work the last time I checked - so get them working.

Edit: A good example of what can be done is Coccinelle used in the Linux kernel which can detect problematic code (locking is way more complex!) as well as apply source transformations. https://www.kernel.org/doc/html/v6.17/dev-tools/coccinelle.h...

[radarsat1]

I agree and honestly it may as well be considered a form of ABI incompatibility. They should make this explicit such that existing C extensions need to be updated to use some new API call for initialization to flag that they are GILless-ready, so that older extensions cannot even successfully be loaded when GIL is disabled.

[rogerbinns]

This has already been done. There is a 't' suffix in the ABI tag.

You have to explicitly compile the extension against a free threaded interpreter in order to get that ABI tag in your extension and even be able to load the extension. The extension then has to opt-in to free threading in its initialization.

If it does not opt-in then a message appears saying the GIL has been enabled, and the interpreter continues to run with the GIL.

This may seem a little strange but is helpful. It means the person running Python doesn't have to keep regular and free threaded Python around, and duplicate sets of extensions etc. They can just have the free threaded one, anything loaded that requires the GIL gives you the normal Python behaviour.

What is a little more problematic is that some of the standard library is marked as supporting free threading, even though they still have the audit and update work outstanding.

Also the last time I checked, the compiler thread sanitizers can't work with free threaded Python.

[electroglyph]

the problem with that is it effects the entire application and makes the whole thing free-threading incompatible.

it's quite possible to make a python app that requires libraries A and B to be able to be loaded into a free-threaded application, but which doesn't actually do any unsafe operations with them. we need to be able to let people load these libraries, but say: this thing may not be safe, add your own mutexes or whatever