diff options
Diffstat (limited to 'Include/object.h')
| -rw-r--r-- | Include/object.h | 87 |
1 files changed, 0 insertions, 87 deletions
diff --git a/Include/object.h b/Include/object.h index 8dccbf1697..4506757d2d 100644 --- a/Include/object.h +++ b/Include/object.h @@ -607,93 +607,6 @@ it carefully, it may save lots of calls to Py_INCREF() and Py_DECREF() at times. */ - -/* Trashcan mechanism, thanks to Christian Tismer. - -When deallocating a container object, it's possible to trigger an unbounded -chain of deallocations, as each Py_DECREF in turn drops the refcount on "the -next" object in the chain to 0. This can easily lead to stack overflows, -especially in threads (which typically have less stack space to work with). - -A container object can avoid this by bracketing the body of its tp_dealloc -function with a pair of macros: - -static void -mytype_dealloc(mytype *p) -{ - ... declarations go here ... - - PyObject_GC_UnTrack(p); // must untrack first - Py_TRASHCAN_BEGIN(p, mytype_dealloc) - ... The body of the deallocator goes here, including all calls ... - ... to Py_DECREF on contained objects. ... - Py_TRASHCAN_END // there should be no code after this -} - -CAUTION: Never return from the middle of the body! If the body needs to -"get out early", put a label immediately before the Py_TRASHCAN_END -call, and goto it. Else the call-depth counter (see below) will stay -above 0 forever, and the trashcan will never get emptied. - -How it works: The BEGIN macro increments a call-depth counter. So long -as this counter is small, the body of the deallocator is run directly without -further ado. But if the counter gets large, it instead adds p to a list of -objects to be deallocated later, skips the body of the deallocator, and -resumes execution after the END macro. The tp_dealloc routine then returns -without deallocating anything (and so unbounded call-stack depth is avoided). - -When the call stack finishes unwinding again, code generated by the END macro -notices this, and calls another routine to deallocate all the objects that -may have been added to the list of deferred deallocations. In effect, a -chain of N deallocations is broken into (N-1)/(PyTrash_UNWIND_LEVEL-1) pieces, -with the call stack never exceeding a depth of PyTrash_UNWIND_LEVEL. - -Since the tp_dealloc of a subclass typically calls the tp_dealloc of the base -class, we need to ensure that the trashcan is only triggered on the tp_dealloc -of the actual class being deallocated. Otherwise we might end up with a -partially-deallocated object. To check this, the tp_dealloc function must be -passed as second argument to Py_TRASHCAN_BEGIN(). -*/ - -/* The new thread-safe private API, invoked by the macros below. */ -PyAPI_FUNC(void) _PyTrash_thread_deposit_object(PyObject*); -PyAPI_FUNC(void) _PyTrash_thread_destroy_chain(void); - -#define PyTrash_UNWIND_LEVEL 50 - -#define Py_TRASHCAN_BEGIN_CONDITION(op, cond) \ - do { \ - PyThreadState *_tstate = NULL; \ - /* If "cond" is false, then _tstate remains NULL and the deallocator \ - * is run normally without involving the trashcan */ \ - if (cond) { \ - _tstate = PyThreadState_GET(); \ - if (_tstate->trash_delete_nesting >= PyTrash_UNWIND_LEVEL) { \ - /* Store the object (to be deallocated later) and jump past \ - * Py_TRASHCAN_END, skipping the body of the deallocator */ \ - _PyTrash_thread_deposit_object(_PyObject_CAST(op)); \ - break; \ - } \ - ++_tstate->trash_delete_nesting; \ - } - /* The body of the deallocator is here. */ -#define Py_TRASHCAN_END \ - if (_tstate) { \ - --_tstate->trash_delete_nesting; \ - if (_tstate->trash_delete_later && _tstate->trash_delete_nesting <= 0) \ - _PyTrash_thread_destroy_chain(); \ - } \ - } while (0); - -#define Py_TRASHCAN_BEGIN(op, dealloc) Py_TRASHCAN_BEGIN_CONDITION(op, \ - Py_TYPE(op)->tp_dealloc == (destructor)(dealloc)) - -/* For backwards compatibility, these macros enable the trashcan - * unconditionally */ -#define Py_TRASHCAN_SAFE_BEGIN(op) Py_TRASHCAN_BEGIN_CONDITION(op, 1) -#define Py_TRASHCAN_SAFE_END(op) Py_TRASHCAN_END - - #ifndef Py_LIMITED_API # define Py_CPYTHON_OBJECT_H # include "cpython/object.h" |