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gh-112075: Iterating a dict shouldn't require locks #115108

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merged 7 commits into from
Feb 22, 2024
Merged

gh-112075: Iterating a dict shouldn't require locks #115108

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afddb70
Thread safe lock-free iteration
DinoV Feb 1, 2024
39bc495
Don't use atomics for iteratio and allow multiple threads to race on …
DinoV Feb 16, 2024
902ce67
Fixup ifdef/ifndef
DinoV Feb 16, 2024
0ba2f5d
Use has_unique_reference
DinoV Feb 16, 2024
042cf26
Less atomics
DinoV Feb 21, 2024
a079a6d
Fix style issues
DinoV Feb 21, 2024
a1d7718
acquire_key_value returns -1 and 0
DinoV Feb 22, 2024
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Don't use atomics for iteratio and allow multiple threads to race on … 
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@DinoV
DinoV committed Feb 22, 2024
commit 39bc495d4dd3ba2e085decd20c6f72e1b65689a1
33 changes: 10 additions & 23 deletions Objects/dictobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5342,7 +5342,7 @@ dictiter_iternext_threadsafe(PyDictObject *d, PyObject *self,

ensure_shared_on_read(d);

Py_ssize_t start_pos = i = _Py_atomic_load_ssize_relaxed(&di->di_pos);
i = _Py_atomic_load_ssize_relaxed(&di->di_pos);
k = _Py_atomic_load_ptr_relaxed(&d->ma_keys);
assert(i >= 0);
if (_PyDict_HasSplitTable(d)) {
Expand Down Expand Up @@ -5402,20 +5402,13 @@ dictiter_iternext_threadsafe(PyDictObject *d, PyObject *self,
}
}
// We found an element (key), but did not expect it
if (_Py_atomic_load_ssize_relaxed(&di->len) == 0) {
Py_ssize_t len;
if ((len = _Py_atomic_load_ssize_relaxed(&di->len)) == 0) {
goto concurrent_modification;
}
if (!_Py_atomic_compare_exchange_ssize(&di->di_pos, &start_pos, i+1)) {
// We lost a race with someone else iterating...
if (out_key != NULL) {
Py_DECREF(*out_key);
}
if (out_value != NULL) {
Py_DECREF(*out_value);
}
goto try_locked;
}
_Py_atomic_add_ssize(&di->len, -1);

_Py_atomic_store_ssize_relaxed(&di->di_pos, i + 1);
_Py_atomic_store_ssize_relaxed(&di->len, len - 1);
return 1;

concurrent_modification:
Expand All @@ -5441,22 +5434,16 @@ static bool
acquire_iter_result(PyObject *result)
{
#ifdef Py_GIL_DISABLED
if (Py_REFCNT(result) == 1) {
Py_INCREF(result);
if (Py_REFCNT(result) == 2) {
return true;
}
Py_DECREF(result);
return false;
}
return false;
if (_Py_IsOwnedByCurrentThread(result) &&
result->ob_ref_local == 1 &&
_Py_atomic_load_ssize_relaxed(&result->ob_ref_shared) == 0) {
#else
if (Py_REFCNT(result) == 1) {
#endif
Py_INCREF(result);
return true;
}
return false;
#endif
}

static PyObject *
Expand Down