Rework FOR_ITER specialization in the free-threaded build to only app…

…ly to uniquely referenced iterators. This handles the common case of 'for item in seq' (where 'seq' is a list, tuple or range object) and 'for item in generator_function()', but not, for example, 'g = gen(...); for item in g:'.
python · Yhg1s · Mar 12, 2025 · Jan 13, 2025 · Jan 13, 2025 · Jan 13, 2025
commit cf5fcd01ec39b5f0e8e57ba5ea24cc70eac5ef62
diff --git a/Include/internal/pycore_list.h b/Include/internal/pycore_list.h
@@ -15,7 +15,7 @@ extern void _PyList_DebugMallocStats(FILE *out);
 extern PyObject* _PyList_GetItemRef(PyListObject *, Py_ssize_t i);
 #ifdef Py_GIL_DISABLED
 // Returns -1 in case of races with other threads.
-extern int _PyList_GetItemRefNoLock(PyListObject *, Py_ssize_t i, PyObject ** result);
+extern int _PyList_GetItemRefNoLock(PyListObject *, Py_ssize_t, PyObject **);
 #endif
 
 #define _PyList_ITEMS(op) _Py_RVALUE(_PyList_CAST(op)->ob_item)

diff --git a/Include/internal/pycore_uop_metadata.h b/Include/internal/pycore_uop_metadata.h
diff --git a/Lib/test/test_free_threading/test_iteration_deopt.py b/Lib/test/test_free_threading/test_iteration_deopt.py
@@ -0,0 +1,155 @@
+import dis
+import queue
+import threading
+import time
+import unittest
+from test.support import (import_helper, cpython_only, Py_GIL_DISABLED,
+                          requires_specialization_ft)
+
+_testinternalcapi = import_helper.import_module("_testinternalcapi")
+_testlimitedcapi = import_helper.import_module("_testlimitedcapi")
+
+NUMTHREADS = 5
+
+def get_tlbc_instructions(f):
+    co = dis._get_code_object(f)
+    tlbc = _testinternalcapi.get_tlbc(co)
+    return [i.opname for i in dis._get_instructions_bytes(tlbc)]
+
+
+class IterationDeoptTests(unittest.TestCase):
+    def check_deopt(self, get_iter, opcode, is_generator=False):
+        input = range(100)
+        expected_len = len(input)
+        q = queue.Queue()
+        barrier = threading.Barrier(NUMTHREADS + 1)
+        done = threading.Event()
+        def worker():
+            # A complicated dance to get a weak reference to an iterator
+            # _only_ (strongly) referenced by the for loop, so that we can
+            # force our loop to deopt mid-way through.
+            it = get_iter(input)
+            ref = _testlimitedcapi.pylong_fromvoidptr(it)
+            q.put(ref)
+            # We can't use enumerate without affecting the loop, so keep a
+            # manual counter.
+            i = 0
+            loop_a_little_more = 5
+            results = []
+            try:
+                # Make sure we're not still specialized from a previous run.
+                ops = get_tlbc_instructions(worker)
+                self.assertIn('FOR_ITER', ops)
+                self.assertNotIn(opcode, ops)
+                for item in it:
+                    results.append(item)
+                    i += 1
+
+                    # We have to be very careful exiting the loop, because
+                    # if the main thread hasn't dereferenced the unsafe
+                    # weakref to our iterator yet, exiting will make it
+                    # invalid and cause a crash. Getting the timing right is
+                    # difficult, though, since it depends on the OS
+                    # scheduler and the system load. As a final safeguard,
+                    # if we're close to finishing the loop, just wait for the
+                    # main thread.
+                    if i + loop_a_little_more > expected_len:
+                        done.wait()
+
+                    if i == 1:
+                        del it
+                    # Warm up. The first iteration didn't count because of
+                    # the extra reference to the iterator.
+                    if i < 10:
+                        continue
+                    if i == 10:
+                        ops = get_tlbc_instructions(worker)
+                        self.assertIn(opcode, ops)
+                        # Let the main thread know it's time to reference our iterator.
+                        barrier.wait()
+                        continue
+                    # Continue iterating while at any time our loop may be
+                    # forced to deopt, but try to get the thread scheduler
+                    # to give the main thread a chance to run.
+                    if not done.is_set():
+                        time.sleep(0)
+                        continue
+                    if loop_a_little_more:
+                        # Loop a little more after 'done' is set to make sure we
+                        # introduce a tsan-detectable race if the loop isn't
+                        # deopting appropriately.
+                        loop_a_little_more -= 1
+                        continue
+                    break
+                self.assertEqual(results, list(input)[:i])
+            except threading.BrokenBarrierError:
+                return
+            except Exception as e:
+                # In case the exception happened before the last barrier,
+                # reset it so nothing is left hanging.
+                barrier.reset()
+                # In case it's the final assertion that failed, just add it
+                # to the result queue so it'll show up in the normal test
+                # output.
+                q.put(e)
+                raise
+            q.put("SUCCESS")
+        # Reset specialization and thread-local bytecode from previous runs.
+        worker.__code__ = worker.__code__.replace()
+        threads = [threading.Thread(target=worker) for i in range(NUMTHREADS)]
+        for t in threads:
+            t.start()
+        # Get the "weakrefs" from the worker threads.
+        refs = [q.get() for i in range(NUMTHREADS)]
+        # Wait for each thread to finish its specialization check.
+        barrier.wait()
+        # Dereference the "weakrefs" we were sent in an extremely unsafe way.
+        iterators = [_testlimitedcapi.pylong_asvoidptr(ref) for ref in refs]
+        done.set()
+        self.assertNotIn(None, iterators)
+        # Read data that the iteration writes, to trigger data races if they
+        # don't deopt appropriately.
+        if is_generator:
+            for it in iterators:
+                it.gi_running
+        else:
+            for it in iterators:
+                it.__reduce__()
+        for t in threads:
+            t.join()
+        results = [q.get() for i in range(NUMTHREADS)]
+        self.assertEqual(results, ["SUCCESS"] * NUMTHREADS)
+
+    @cpython_only
+    @requires_specialization_ft
+    @unittest.skipIf(not Py_GIL_DISABLED, "requires free-threading")
+    def test_deopt_leaking_iterator_list(self):
+        def make_list_iter(input):
+            return iter(list(input))
+        self.check_deopt(make_list_iter, 'FOR_ITER_LIST')
+
+    @cpython_only
+    @requires_specialization_ft
+    @unittest.skipIf(not Py_GIL_DISABLED, "requires free-threading")
+    def test_deopt_leaking_iterator_tuple(self):
+        def make_tuple_iter(input):
+            return iter(tuple(input))
+        self.check_deopt(make_tuple_iter, 'FOR_ITER_TUPLE')
+
+    @cpython_only
+    @requires_specialization_ft
+    @unittest.skipIf(not Py_GIL_DISABLED, "requires free-threading")
+    def test_deopt_leaking_iterator_range(self):
+        def make_range_iter(input):
+            return iter(input)
+        self.check_deopt(make_range_iter, 'FOR_ITER_RANGE')
+
+    @cpython_only
+    @requires_specialization_ft
+    @unittest.skipIf(not Py_GIL_DISABLED, "requires free-threading")
+    def test_deopt_leaking_iterator_generator(self):
+        def gen(input):
+            for item in input:
+                yield item
+        self.check_deopt(gen, 'FOR_ITER_GEN', is_generator=True)
+
diff --git a/Lib/test/test_opcache.py b/Lib/test/test_opcache.py
@@ -1631,6 +1631,44 @@ def compare_op_str():
         self.assert_specialized(compare_op_str, "COMPARE_OP_STR")
         self.assert_no_opcode(compare_op_str, "COMPARE_OP")
 
+    @cpython_only
+    @requires_specialization_ft
+    def test_for_iter(self):
+        L = list(range(10))
+        def for_iter_list():
+            for i in L:
+                self.assertIn(i, L)
+
+        for_iter_list()
+        self.assert_specialized(for_iter_list, "FOR_ITER_LIST")
+        self.assert_no_opcode(for_iter_list, "FOR_ITER")
+
+        t = tuple(range(10))
+        def for_iter_tuple():
+            for i in t:
+                self.assertIn(i, t)
+
+        for_iter_tuple()
+        self.assert_specialized(for_iter_tuple, "FOR_ITER_TUPLE")
+        self.assert_no_opcode(for_iter_tuple, "FOR_ITER")
+
+        r = range(10)
+        def for_iter_range():
+            for i in r:
+                self.assertIn(i, r)
+
+        for_iter_range()
+        self.assert_specialized(for_iter_range, "FOR_ITER_RANGE")
+        self.assert_no_opcode(for_iter_range, "FOR_ITER")
+
+        def for_iter_generator():
+            for i in (i for i in range(10)):
+                i + 1
+
+        for_iter_generator()
+        self.assert_specialized(for_iter_generator, "FOR_ITER_GEN")
+        self.assert_no_opcode(for_iter_generator, "FOR_ITER")
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Objects/rangeobject.c b/Objects/rangeobject.c
@@ -7,7 +7,6 @@
 #include "pycore_modsupport.h"    // _PyArg_NoKwnames()
 #include "pycore_range.h"
 #include "pycore_tuple.h"         // _PyTuple_ITEMS()
-#include "pycore_pyatomic_ft_wrappers.h"
 
 
 /* Support objects whose length is > PY_SSIZE_T_MAX.
@@ -817,12 +816,10 @@ PyTypeObject PyRange_Type = {
 static PyObject *
 rangeiter_next(_PyRangeIterObject *r)
 {
-    long len = FT_ATOMIC_LOAD_LONG_RELAXED(r->len);
-    if (len > 0) {
-        long result = FT_ATOMIC_LOAD_LONG_RELAXED(r->start);
-        FT_ATOMIC_STORE_LONG_RELAXED(r->start, result + r->step);
-        // Relaxed ops for maximum speed and minimum thread-safety.
-        FT_ATOMIC_STORE_LONG_RELAXED(r->len, len - 1);
+    if (r->len > 0) {
+        long result = r->start;
+        r->start = result + r->step;
+        r->len--;
         return PyLong_FromLong(result);
     }
     return NULL;
@@ -831,7 +828,7 @@ rangeiter_next(_PyRangeIterObject *r)
 static PyObject *
 rangeiter_len(_PyRangeIterObject *r, PyObject *Py_UNUSED(ignored))
 {
-    return PyLong_FromLong(FT_ATOMIC_LOAD_LONG_RELAXED(r->len));
+    return PyLong_FromLong(r->len);
 }
 
 PyDoc_STRVAR(length_hint_doc,
@@ -844,11 +841,10 @@ rangeiter_reduce(_PyRangeIterObject *r, PyObject *Py_UNUSED(ignored))
     PyObject *range;
 
     /* create a range object for pickling */
-    long lstart = FT_ATOMIC_LOAD_LONG_RELAXED(r->start);
-    start = PyLong_FromLong(lstart);
+    start = PyLong_FromLong(r->start);
     if (start == NULL)
         goto err;
-    stop = PyLong_FromLong(lstart + FT_ATOMIC_LOAD_LONG_RELAXED(r->len) * r->step);
+    stop = PyLong_FromLong(r->start + r->len * r->step);
     if (stop == NULL)
         goto err;
     step = PyLong_FromLong(r->step);
@@ -875,14 +871,12 @@ rangeiter_setstate(_PyRangeIterObject *r, PyObject *state)
     if (index == -1 && PyErr_Occurred())
         return NULL;
     /* silently clip the index value */
-    long len = FT_ATOMIC_LOAD_LONG_RELAXED(r->len);
     if (index < 0)
         index = 0;
-    else if (index > len)
-        index = len; /* exhausted iterator */
-    FT_ATOMIC_STORE_LONG_RELAXED(r->start,
-        FT_ATOMIC_LOAD_LONG_RELAXED(r->start) + index * r->step);
-    FT_ATOMIC_STORE_LONG_RELAXED(r->len, len - index);
+    else if (index > r->len)
+        index = r->len; /* exhausted iterator */
+    r->start += index * r->step;
+    r->len -= index;
     Py_RETURN_NONE;
 }
 

diff --git a/Objects/tupleobject.c b/Objects/tupleobject.c
@@ -1020,15 +1020,13 @@ tupleiter_next(PyObject *self)
         return NULL;
     assert(PyTuple_Check(seq));
 
-    Py_ssize_t idx = FT_ATOMIC_LOAD_SSIZE_RELAXED(it->it_index);
-    if ((size_t)idx < (size_t)PyTuple_GET_SIZE(seq)) {
-        item = PyTuple_GET_ITEM(seq, idx++);
-        FT_ATOMIC_STORE_SSIZE_RELAXED(it->it_index, idx);
+    if (it->it_index < PyTuple_GET_SIZE(seq)) {
+        item = PyTuple_GET_ITEM(seq, it->it_index);
+        ++it->it_index;
         return Py_NewRef(item);
     }
 
-    FT_ATOMIC_STORE_SSIZE_RELAXED(it->it_index, -1);
-    FT_ATOMIC_STORE_PTR_RELAXED(it->it_seq, NULL);
+    it->it_seq = NULL;
     Py_DECREF(seq);
     return NULL;
 }