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gh-117657: Fix race involving GC and heap initialization (#119923)

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The `_PyThreadState_Bind()` function is called before the first
`PyEval_AcquireThread()` so it's not synchronized with the stop the
world GC. We had a race where `gc_visit_heaps()` might visit a thread's
heap while it's being initialized.

Use a simple atomic int to avoid visiting heaps for threads that are not
yet fully initialized (i.e., before `tstate_mimalloc_bind()` is called).

The race was reproducible by running:
`python Lib/test/test_importlib/partial/pool_in_threads.py`.
This commit is contained in:
Sam Gross 2024-06-04 09:42:13 -04:00 • committed by GitHub
parent bd8c1f97e1
commit e69d068ad0
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4 changed files with 7 additions and 3 deletions
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4
Python/gc_free_threading.c
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@ -252,6 +252,10 @@ gc_visit_heaps_lock_held(PyInterpreterState *interp, mi_block_visit_fun *visitor
// visit each thread's heaps for GC objects
for (PyThreadState *p = interp->threads.head; p != NULL; p = p->next) {
struct _mimalloc_thread_state *m = &((_PyThreadStateImpl *)p)->mimalloc;
if (!_Py_atomic_load_int(&m->initialized)) {
// The thread may not have called tstate_mimalloc_bind() yet.
continue;
}
arg->offset = offset_base;
if (!mi_heap_visit_blocks(&m->heaps[_Py_MIMALLOC_HEAP_GC], true,

2
Python/pystate.c
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@ -3074,6 +3074,8 @@ tstate_mimalloc_bind(PyThreadState *tstate)
// _PyObject_GC_New() and similar functions temporarily override this to
// use one of the GC heaps.
mts->current_object_heap = &mts->heaps[_Py_MIMALLOC_HEAP_OBJECT];
_Py_atomic_store_int(&mts->initialized, 1);
#endif
}