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gh-124375: Avoid calling `_PyMem_ProcessDelayed` on other thread states (#124459) 

This fixes a crash when running the PyO3 test suite on the free-threaded
build. The `qsbr` field is initialized after the `PyThreadState` is
added to the interpreter's linked list -- it might still be NULL.

Instead, we "steal" the queue of to-be-freed memory blocks. This is
always initialized (possibly empty) and protected by the stop the world
pause.
This commit is contained in:
Sam Gross 2024-10-15 12:09:35 -04:00 committed by GitHub
parent e97910cdb7
commit 54c6fcbefd
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2 changed files with 13 additions and 6 deletions
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Misc/NEWS.d/next/Core_and_Builtins/2024-10-15-15-29-41.gh-issue-124375.wNrWVa.rst Normal file
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@ -0,0 +1 @@
Fix a crash in the free threading build when the GC runs concurrently with a new thread starting.

18
Python/gc_free_threading.c
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@ -420,18 +420,24 @@ merge_queued_objects(_PyThreadStateImpl *tstate, struct collection_state *state)
static void
process_delayed_frees(PyInterpreterState *interp)
{
// In STW status, we can observe the latest write sequence by
// advancing the write sequence immediately.
// While we are in a "stop the world" pause, we can observe the latest
// write sequence by advancing the write sequence immediately.
_Py_qsbr_advance(&interp->qsbr);
_PyThreadStateImpl *current_tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
_Py_qsbr_quiescent_state(current_tstate->qsbr);
// Merge the queues from other threads into our own queue so that we can
// process all of the pending delayed free requests at once.
HEAD_LOCK(&_PyRuntime);
PyThreadState *tstate = interp->threads.head;
while (tstate != NULL) {
_PyMem_ProcessDelayed(tstate);
tstate = (PyThreadState *)tstate->next;
for (PyThreadState *p = interp->threads.head; p != NULL; p = p->next) {
_PyThreadStateImpl *other = (_PyThreadStateImpl *)p;
if (other != current_tstate) {
llist_concat(&current_tstate->mem_free_queue, &other->mem_free_queue);
}
}
HEAD_UNLOCK(&_PyRuntime);
_PyMem_ProcessDelayed((PyThreadState *)current_tstate);
}
// Subtract an incoming reference from the computed "gc_refs" refcount.