Type slots are not thread-safe in free-threaded builds

@colesbury

Bug report

Bug description:

Modification of type slots is protected by the global type lock, however, type slots are read non-atomically without holding the type lock. For example, in PyObject_SetItem:

cpython/Objects/abstract.c

Lines 231 to 235 in 5bb059f

    
           if (m && m->mp_ass_subscript) { 
        
               int res = m->mp_ass_subscript(o, key, value); 
        
               assert(_Py_CheckSlotResult(o, "__setitem__", res >= 0)); 
        
               return res; 
        
           }

It's not clear how we want to address this. From @colesbury in #127169 (comment):

I'd lean towards doing a stop-the-world pause when modifying the slot so that we don't need 
to change every read. I expect that to be a bit tricky since class definitions look like they're mutating 
the class.

CPython versions tested on:

3.13, 3.14, CPython main branch

Operating systems tested on:

Linux

Linked PRs

For future reference, you can reproduce this race by running the following test with TSAN enabled:

./python -m test test_opcache -m test_load_attr_method_lazy_dict

pythongh-117657: Enable test_opcache under TSAN

gh-117657: Enable test_opcache under TSAN (GH-129831)

[3.13] pythongh-117657: Enable test_opcache under TSAN (pythonGH-129831)

[3.13] gh-117657: Enable test_opcache under TSAN (GH-129831) (GH-130597)

Merge 'origin/main' into pythongh-127266-type-slots-ts

Merge branch 'origin/main' into pythongh-127266-type-slots-ts

Merge 'origin/main' into pythongh-127266-type-slots-ts

gh-127266: avoid data races when updating type slots (gh-131174)

Revert pythongh-127266: avoid data races when updating type slots (py…

Revert gh-127266: avoid data races when updating type slots (gh-131174)…

pythongh-127266: avoid data races when updating type slots

gh-127266: avoid data races when updating type slots (gh-133177)

pythongh-127266: avoid data races when updating type slots (pythongh-…

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Type slots are not thread-safe in free-threaded builds #127266

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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	if (m && m->mp_ass_subscript) {
	int res = m->mp_ass_subscript(o, key, value);
	assert(_Py_CheckSlotResult(o, "__setitem__", res >= 0));
	return res;
	}

Type slots are not thread-safe in free-threaded builds #127266

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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Pitmbar commented on Dec 5, 2024

colesbury commented on Feb 7, 2025

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