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Merged
colesbury merged 3 commits into from
Dec 11, 2025
Merged

gh-142534: Avoid TSan warnings in dictobject.c #142544

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0dae3ba
gh-142534: Avoid TSan warnings in dictobject.c
colesbury Dec 10, 2025
ff0373a
Add missing define
colesbury Dec 11, 2025
536c3cb
Remove stray whitespace change
colesbury Dec 11, 2025
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gh-142534: Avoid TSan warnings in dictobject.c 
There are places we use "relaxed" loads where C11 requires "consume" or
stronger. Unfortunately, compilers don't really implement "consume" so
fake it for our use in a way that avoids upsetting TSan.
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@colesbury
colesbury committed Dec 10, 2025
commit 0dae3baeb9775fc4fcb913ff2bf305e10c190362
11 changes: 11 additions & 0 deletions Include/cpython/pyatomic.h
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Expand Up @@ -591,6 +591,17 @@ static inline void _Py_atomic_fence_release(void);

// --- aliases ---------------------------------------------------------------

// Compilers don't really support "consume" semantics, so we fake it. Use
// "acquire" with TSan to support false positives. Use "relaxed" otherwise,
// because CPUs on all platforms we support respect address dependencies without
// extra barriers.
// See 2.6.7 in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2055r0.pdf
#if defined(_Py_THREAD_SANITIZER)
# define _Py_atomic_load_ptr_consume _Py_atomic_load_ptr_acquire
#else
# define _Py_atomic_load_ptr_consume _Py_atomic_load_ptr_relaxed
#endif

#if SIZEOF_LONG == 8
# define _Py_atomic_load_ulong(p) \
_Py_atomic_load_uint64((uint64_t *)p)
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1 change: 0 additions & 1 deletion Include/cpython/pyatomic_gcc.h
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Expand Up @@ -549,7 +549,6 @@ static inline void
_Py_atomic_store_llong_relaxed(long long *obj, long long value)
{ __atomic_store_n(obj, value, __ATOMIC_RELAXED); }


// --- _Py_atomic_load_ptr_acquire / _Py_atomic_store_ptr_release ------------

static inline void *
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2 changes: 2 additions & 0 deletions Include/internal/pycore_pyatomic_ft_wrappers.h
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Expand Up @@ -31,6 +31,8 @@ extern "C" {
_Py_atomic_store_ptr(&value, new_value)
#define FT_ATOMIC_LOAD_PTR_ACQUIRE(value) \
_Py_atomic_load_ptr_acquire(&value)
#define FT_ATOMIC_LOAD_PTR_CONSUME(value) \
_Py_atomic_load_ptr_consume(&value)
#define FT_ATOMIC_LOAD_UINTPTR_ACQUIRE(value) \
_Py_atomic_load_uintptr_acquire(&value)
#define FT_ATOMIC_LOAD_PTR_RELAXED(value) \
Expand Down
12 changes: 6 additions & 6 deletions Objects/dictobject.c
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Expand Up @@ -1078,7 +1078,7 @@
void *ep0, Py_ssize_t ix, PyObject *key, Py_hash_t hash)
{
PyDictUnicodeEntry *ep = &((PyDictUnicodeEntry *)ep0)[ix];
PyObject *ep_key = FT_ATOMIC_LOAD_PTR_RELAXED(ep->me_key);
PyObject *ep_key = FT_ATOMIC_LOAD_PTR_CONSUME(ep->me_key);

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Windows / Build and test (x64) 

'initializing': 'PyObject *' differs in levels of indirection from 'int' [D:\a\cpython\cpython\PCbuild\_freeze_module.vcxproj]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Windows / Build and test (x64) 

'FT_ATOMIC_LOAD_PTR_CONSUME' undefined; assuming extern returning int [D:\a\cpython\cpython\PCbuild\_freeze_module.vcxproj]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Hypothesis tests on Ubuntu 

initialization of ‘PyObject *’ {aka ‘struct _object *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

Check failure on line 1081 in Objects/dictobject.c

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GitHub Actions / Hypothesis tests on Ubuntu 

implicit declaration of function ‘FT_ATOMIC_LOAD_PTR_CONSUME’; did you mean ‘FT_ATOMIC_LOAD_PTR_ACQUIRE’? [-Werror=implicit-function-declaration]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Cross build Linux 

initialization of ‘PyObject *’ {aka ‘struct _object *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

Check failure on line 1081 in Objects/dictobject.c

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GitHub Actions / Cross build Linux 

implicit declaration of function ‘FT_ATOMIC_LOAD_PTR_CONSUME’; did you mean ‘FT_ATOMIC_LOAD_PTR_ACQUIRE’? [-Werror=implicit-function-declaration]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Windows / Build and test (arm64) 

'initializing': 'PyObject *' differs in levels of indirection from 'int' [C:\a\cpython\cpython\PCbuild\_freeze_module.vcxproj]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Windows / Build and test (arm64) 

'FT_ATOMIC_LOAD_PTR_CONSUME' undefined; assuming extern returning int [C:\a\cpython\cpython\PCbuild\_freeze_module.vcxproj]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Address sanitizer (ubuntu-24.04) 

initialization of ‘PyObject *’ {aka ‘struct _object *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

Check failure on line 1081 in Objects/dictobject.c

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GitHub Actions / Address sanitizer (ubuntu-24.04) 

implicit declaration of function ‘FT_ATOMIC_LOAD_PTR_CONSUME’; did you mean ‘FT_ATOMIC_LOAD_PTR_ACQUIRE’? [-Werror=implicit-function-declaration]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Ubuntu / build and test (ubuntu-24.04) 

initialization of ‘PyObject *’ {aka ‘struct _object *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

Check failure on line 1081 in Objects/dictobject.c

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GitHub Actions / Ubuntu / build and test (ubuntu-24.04) 

implicit declaration of function ‘FT_ATOMIC_LOAD_PTR_CONSUME’; did you mean ‘FT_ATOMIC_LOAD_PTR_ACQUIRE’? [-Werror=implicit-function-declaration]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Ubuntu / build and test (ubuntu-24.04-arm) 

initialization of ‘PyObject *’ {aka ‘struct _object *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

Check failure on line 1081 in Objects/dictobject.c

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GitHub Actions / Ubuntu / build and test (ubuntu-24.04-arm) 

implicit declaration of function ‘FT_ATOMIC_LOAD_PTR_CONSUME’; did you mean ‘FT_ATOMIC_LOAD_PTR_ACQUIRE’? [-Werror=implicit-function-declaration]

Check warning on line 1081 in Objects/dictobject.c

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GitHub Actions / Ubuntu (bolt) / build and test (ubuntu-24.04) 

initialization of ‘PyObject *’ {aka ‘struct _object *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

Check failure on line 1081 in Objects/dictobject.c

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GitHub Actions / Ubuntu (bolt) / build and test (ubuntu-24.04) 

implicit declaration of function ‘FT_ATOMIC_LOAD_PTR_CONSUME’; did you mean ‘FT_ATOMIC_LOAD_PTR_ACQUIRE’? [-Werror=implicit-function-declaration]
assert(ep_key != NULL);
assert(PyUnicode_CheckExact(ep_key));
if (ep_key == key ||
Expand Down Expand Up @@ -1371,7 +1371,7 @@
void *ep0, Py_ssize_t ix, PyObject *key, Py_hash_t hash)
{
PyDictUnicodeEntry *ep = &((PyDictUnicodeEntry *)ep0)[ix];
PyObject *startkey = _Py_atomic_load_ptr_relaxed(&ep->me_key);
PyObject *startkey = _Py_atomic_load_ptr_consume(&ep->me_key);
assert(startkey == NULL || PyUnicode_CheckExact(ep->me_key));
assert(!PyUnicode_CheckExact(key));

Expand Down Expand Up @@ -1414,7 +1414,7 @@
void *ep0, Py_ssize_t ix, PyObject *key, Py_hash_t hash)
{
PyDictUnicodeEntry *ep = &((PyDictUnicodeEntry *)ep0)[ix];
PyObject *startkey = _Py_atomic_load_ptr_relaxed(&ep->me_key);
PyObject *startkey = _Py_atomic_load_ptr_consume(&ep->me_key);
if (startkey == key) {
assert(PyUnicode_CheckExact(startkey));
return 1;
Expand Down Expand Up @@ -1450,7 +1450,7 @@
void *ep0, Py_ssize_t ix, PyObject *key, Py_hash_t hash)
{
PyDictKeyEntry *ep = &((PyDictKeyEntry *)ep0)[ix];
PyObject *startkey = _Py_atomic_load_ptr_relaxed(&ep->me_key);
PyObject *startkey = _Py_atomic_load_ptr_consume(&ep->me_key);
if (startkey == key) {
return 1;
}
Expand Down Expand Up @@ -5526,7 +5526,7 @@
k = _Py_atomic_load_ptr_acquire(&d->ma_keys);
assert(i >= 0);
if (_PyDict_HasSplitTable(d)) {
PyDictValues *values = _Py_atomic_load_ptr_relaxed(&d->ma_values);
PyDictValues *values = _Py_atomic_load_ptr_consume(&d->ma_values);
if (values == NULL) {
goto concurrent_modification;
}
Expand Down Expand Up @@ -7114,7 +7114,7 @@
Py_BEGIN_CRITICAL_SECTION(dict);

if (dict->ma_values == values && FT_ATOMIC_LOAD_UINT8(values->valid)) {
value = _Py_atomic_load_ptr_relaxed(&values->values[ix]);
value = _Py_atomic_load_ptr_consume(&values->values[ix]);
*attr = _Py_XNewRefWithLock(value);
success = true;
} else {
Expand Down
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