8000 gh-112075: Iterating a dict shouldn't require locks by DinoV · Pull Request #115108 · python/cpython · GitHub
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Merged
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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Less atomics
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@DinoV
DinoV committed Feb 22, 2024
commit 042cf26bb76753f145f1f8ef42c5705bf1899f91
52 changes: 18 additions & 34 deletions Objects/dictobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -115,19 +115,20 @@ As a consequence of this, split keys have a maximum size of 16.
#define PyDict_MINSIZE 8

#include "Python.h"
#include "pycore_bitutils.h" // _Py_bit_length
#include "pycore_call.h" // _PyObject_CallNoArgs()
#include "pycore_ceval.h" // _PyEval_GetBuiltin()
#include "pycore_code.h" // stats
#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION, Py_END_CRITICAL_SECTION
#include "pycore_dict.h" // export _PyDict_SizeOf()
#include "pycore_freelist.h" // _PyFreeListState_GET()
#include "pycore_gc.h" // _PyObject_GC_IS_TRACKED()
#include "pycore_object.h" // _PyObject_GC_TRACK(), _PyDebugAllocatorStats()
#include "pycore_pyerrors.h" // _PyErr_GetRaisedException()
#include "pycore_pystate.h" // _PyThreadState_GET()
#include "pycore_setobject.h" // _PySet_NextEntry()
#include "stringlib/eq.h" // unicode_eq()
#include "pycore_bitutils.h" // _Py_bit_length
#include "pycore_call.h" // _PyObject_CallNoArgs()
#include "pycore_ceval.h" // _PyEval_GetBuiltin()
#include "pycore_code.h" // stats
#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION, Py_END_CRITICAL_SECTION
#include "pycore_dict.h" // export _PyDict_SizeOf()
#include "pycore_freelist.h" // _PyFreeListState_GET()
#include "pycore_gc.h" // _PyObject_GC_IS_TRACKED()
#include "pycore_object.h" // _PyObject_GC_TRACK(), _PyDebugAllocatorStats()
#include "pycore_pyatomic_ft_wrappers.h" // FT_ATOMIC_LOAD_SSIZE_RELAXED
#include "pycore_pyerrors.h" // _PyErr_GetRaisedException()
#include "pycore_pystate.h" // _PyThreadState_GET()
#include "pycore_setobject.h" // _PySet_NextEntry()
#include "stringlib/eq.h" // unicode_eq()

#include <stdbool.h>

Expand Down Expand Up @@ -159,7 +160,6 @@ ASSERT_DICT_LOCKED(PyObject *op)
#define ASSERT_OWNED_OR_SHARED(mp) \
assert(_Py_IsOwnedByCurrentThread((PyObject *)mp) || IS_DICT_SHARED(mp));
#define LOAD_KEYS_NENTRIES(d)
#define DECREMENT(v) _Py_atomic_add_ssize(&(v), -1)

static inline Py_ssize_t
load_keys_nentries(PyDictObject *mp)
Expand Down Expand Up @@ -222,7 +222,6 @@ static inline void split_keys_entry_added(PyDictKeysObject *keys)
#define SET_DICT_SHARED(mp)
#define LOAD_INDEX(keys, size, idx) ((const int##size##_t*)(keys->dk_indices))[idx]
#define STORE_INDEX(keys, size, idx, value) ((int##size##_t*)(keys->dk_indices))[idx] = (int##size##_t)value
#define DECREMENT(v) ((v)--)

static inline void split_keys_entry_added(PyDictKeysObject *keys)
{
Expand Down Expand Up @@ -5017,7 +5016,7 @@ dictiter_iternextkey_lock_held(PyDictObject *d, PyObject *self)
return NULL;
}

#endif
#endif /* Py_GIL_DISABLED */

static PyObject*
dictiter_iternextkey(PyObject *self)
Expand Down Expand Up @@ -5140,7 +5139,7 @@ dictiter_iternextvalue_lock_held(PyDictObject *d, PyObject *self)
return NULL;
}

#endif
#endif /* Py_GIL_DISABLED */

static PyObject *
dictiter_iternextvalue(PyObject *self)
Expand Down Expand Up @@ -5213,15 +5212,7 @@ dictiter_iternextitem_lock_held(PyDictObject *d, PyObject *self,
return 0;
}

#ifdef Py_GIL_DISABLED
Py_ssize_t start_pos;
// Even though we hold the lock here we may still lose a race against
// a lock-free iterator, therefore we may end up retrying our iteration.
retry:
start_pos = i = _Py_atomic_load_ssize_relaxed(&di->di_pos);
#else
i = di->di_pos;
#endif
i = FT_ATOMIC_LOAD_SSIZE_RELAXED(di->di_pos);

assert(i >= 0);
if (_PyDict_HasSplitTable(d)) {
Expand Down Expand Up @@ -5263,15 +5254,8 @@ dictiter_iternextitem_lock_held(PyDictObject *d, PyObject *self,
"dictionary keys changed during iteration");
goto fail;
}
#ifdef Py_GIL_DISABLED
if (!_Py_atomic_compare_exchange_ssize(&di->di_pos, &start_pos, i+1)) {
// We lost a a race with a lock-free iterator!
goto retry;
}
#else
di->di_pos = i+1;
#endif
DECREMENT(di->len);
di->len--;
if (out_key != NULL) {
*out_key = Py_NewRef(key);
}
Expand Down
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