GeezerVisonThe World Wide Web, restored to 800 pixels of glory. |
| Home|About|Gaming cabinet|Archives|How it works|Starter sites |
GeezerVison is showing github.com — cpython/Python/gc.c at main · python/cpython Expand file tree/ Copy pathgc.cMore file actions 2237 lines (1983 loc) · 75.1 KB / Copy pathgc.cFile metadata and controls2237 lines (1983 loc) · 75.1 KB 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 // This implements the reference cycle garbage collector. // The Python module interface to the collector is in gcmodule.c. // See InternalDocs/garbage_collector.md for more infromation. #include "Python.h" #include "pycore_ceval.h" // _Py_set_eval_breaker_bit() #include "pycore_dict.h" // _PyInlineValuesSize() #include "pycore_genobject.h" // _PyGen_FinalizerIsNoop() #include "pycore_initconfig.h" // _PyStatus_OK() #include "pycore_context.h" #include "pycore_interp.h" // PyInterpreterState.gc #include "pycore_interpframe.h" // _PyFrame_GetLocalsArray() #include "pycore_object.h" #include "pycore_object_alloc.h" // _PyObject_MallocWithType() #include "pycore_pyerrors.h" #include "pycore_pystate.h" // _PyThreadState_GET() #include "pycore_tuple.h" // _PyTuple_MaybeUntrack() #include "pycore_weakref.h" // _PyWeakref_ClearRef() #include "pydtrace.h" #if !defined(Py_GIL_DISABLED) typedef struct _gc_runtime_state GCState; #ifdef Py_DEBUG # define GC_DEBUG #endif #define GC_NEXT _PyGCHead_NEXT #define GC_PREV _PyGCHead_PREV // update_refs() set this bit for all objects in current generation. // subtract_refs() and move_unreachable() uses this to distinguish // visited object is in GCing or not. // // move_unreachable() removes this flag from reachable objects. // Only unreachable objects have this flag. // // No objects in interpreter have this flag after GC ends. #define PREV_MASK_COLLECTING _PyGC_PREV_MASK_COLLECTING // Lowest bit of _gc_next is used for UNREACHABLE flag. // // This flag represents the object is in unreachable list in move_unreachable() // // Although this flag is used only in move_unreachable(), move_unreachable() // doesn't clear this flag to skip unnecessary iteration. // move_legacy_finalizers() removes this flag instead. // Between them, unreachable list is not normal list and we can not use // most gc_list_* functions for it. #define NEXT_MASK_UNREACHABLE (1) #define AS_GC(op) _Py_AS_GC(op) #define FROM_GC(gc) _Py_FROM_GC(gc) // Automatically choose the generation that needs collecting. #define GENERATION_AUTO (-1) static inline int gc_is_collecting(PyGC_Head *g) { return (g->_gc_prev & PREV_MASK_COLLECTING) != 0; } static inline void gc_clear_collecting(PyGC_Head *g) { g->_gc_prev &= ~PREV_MASK_COLLECTING; } static inline Py_ssize_t gc_get_refs(PyGC_Head *g) { return (Py_ssize_t)(g->_gc_prev >> _PyGC_PREV_SHIFT); } static inline void gc_set_refs(PyGC_Head *g, Py_ssize_t refs) { g->_gc_prev = (g->_gc_prev & ~_PyGC_PREV_MASK) | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT); } static inline void gc_reset_refs(PyGC_Head *g, Py_ssize_t refs) { g->_gc_prev = (g->_gc_prev & _PyGC_PREV_MASK_FINALIZED) | PREV_MASK_COLLECTING | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT); } static inline void gc_decref(PyGC_Head *g) { _PyObject_ASSERT_WITH_MSG(FROM_GC(g), gc_get_refs(g) > 0, "refcount is too small"); g->_gc_prev -= 1 << _PyGC_PREV_SHIFT; } #define GEN_HEAD(gcstate, n) (&(gcstate)->generations[n].head) static GCState * get_gc_state(void) { PyInterpreterState *interp = _PyInterpreterState_GET(); return &interp->gc; } void _PyGC_InitState(GCState *gcstate) { #define INIT_HEAD(GEN) \ do { \ GEN.head._gc_next = (uintptr_t)&GEN.head; \ GEN.head._gc_prev = (uintptr_t)&GEN.head; \ } while (0) for (int i = 0; i < NUM_GENERATIONS; i++) { assert(gcstate->generations[i].count == 0); INIT_HEAD(gcstate->generations[i]); }; gcstate->generation0 = GEN_HEAD(gcstate, 0); INIT_HEAD(gcstate->permanent_generation); #undef INIT_HEAD } PyStatus _PyGC_Init(PyInterpreterState *interp) { GCState *gcstate = &interp->gc; gcstate->generation_stats = PyMem_RawCalloc(1, sizeof(struct gc_stats)); if (gcstate->generation_stats == NULL) { return _PyStatus_NO_MEMORY(); } gcstate->garbage = PyList_New(0); if (gcstate->garbage == NULL) { return _PyStatus_NO_MEMORY(); } gcstate->callbacks = PyList_New(0); if (gcstate->callbacks == NULL) { return _PyStatus_NO_MEMORY(); } return _PyStatus_OK(); } /* _gc_prev values --------------- Between collections, _gc_prev is used for doubly linked list. Lowest two bits of _gc_prev are used for flags. PREV_MASK_COLLECTING is used only while collecting and cleared before GC ends or _PyObject_GC_UNTRACK() is called. During a collection, _gc_prev is temporary used for gc_refs, and the gc list is singly linked until _gc_prev is restored. gc_refs At the start of a collection, update_refs() copies the true refcount to gc_refs, for each object in the generation being collected. subtract_refs() then adjusts gc_refs so that it equals the number of times an object is referenced directly from outside the generation being collected. PREV_MASK_COLLECTING Objects in generation being collected are marked PREV_MASK_COLLECTING in update_refs(). _gc_next values --------------- _gc_next takes these values: 0 The object is not tracked != 0 Pointer to the next object in the GC list. Additionally, lowest bit is used temporary for NEXT_MASK_UNREACHABLE flag described below. NEXT_MASK_UNREACHABLE move_unreachable() then moves objects not reachable (whether directly or indirectly) from outside the generation into an "unreachable" set and set this flag. Objects that are found to be reachable have gc_refs set to 1. When this flag is set for the reachable object, the object must be in "unreachable" set. The flag is unset and the object is moved back to "reachable" set. move_legacy_finalizers() will remove this flag from "unreachable" set. */ /*** list functions ***/ static inline void gc_list_init(PyGC_Head *list) { // List header must not have flags. // We can assign pointer by simple cast. list->_gc_prev = (uintptr_t)list; list->_gc_next = (uintptr_t)list; } static inline int gc_list_is_empty(PyGC_Head *list) { return (list->_gc_next == (uintptr_t)list); } /* Append `node` to `list`. */ static inline void gc_list_append(PyGC_Head *node, PyGC_Head *list) { PyGC_Head *last = (PyGC_Head *)list->_gc_prev; // last <-> node _PyGCHead_SET_PREV(node, last); _PyGCHead_SET_NEXT(last, node); // node <-> list _PyGCHead_SET_NEXT(node, list); list->_gc_prev = (uintptr_t)node; } /* Remove `node` from the gc list it's currently in. */ static inline void gc_list_remove(PyGC_Head *node) { PyGC_Head *prev = GC_PREV(node); PyGC_Head *next = GC_NEXT(node); _PyGCHead_SET_NEXT(prev, next); _PyGCHead_SET_PREV(next, prev); node->_gc_next = 0; /* object is not currently tracked */ } /* Move `node` from the gc list it's currently in (which is not explicitly * named here) to the end of `list`. This is semantically the same as * gc_list_remove(node) followed by gc_list_append(node, list). */ static void gc_list_move(PyGC_Head *node, PyGC_Head *list) { /* Unlink from current list. */ PyGC_Head *from_prev = GC_PREV(node); PyGC_Head *from_next = GC_NEXT(node); _PyGCHead_SET_NEXT(from_prev, from_next); _PyGCHead_SET_PREV(from_next, from_prev); /* Relink at end of new list. */ // list must not have flags. So we can skip macros. PyGC_Head *to_prev = (PyGC_Head*)list->_gc_prev; _PyGCHead_SET_PREV(node, to_prev); _PyGCHead_SET_NEXT(to_prev, node); list->_gc_prev = (uintptr_t)node; _PyGCHead_SET_NEXT(node, list); } /* append list `from` onto list `to`; `from` becomes an empty list */ static void gc_list_merge(PyGC_Head *from, PyGC_Head *to) { assert(from != to); if (!gc_list_is_empty(from)) { PyGC_Head *to_tail = GC_PREV(to); PyGC_Head *from_head = GC_NEXT(from); PyGC_Head *from_tail = GC_PREV(from); assert(from_head != from); assert(from_tail != from); _PyGCHead_SET_NEXT(to_tail, from_head); _PyGCHead_SET_PREV(from_head, to_tail); _PyGCHead_SET_NEXT(from_tail, to); _PyGCHead_SET_PREV(to, from_tail); } gc_list_init(from); } static Py_ssize_t gc_list_size(PyGC_Head *list) { PyGC_Head *gc; Py_ssize_t n = 0; for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) { n++; } return n; } /* Walk the list and mark all objects as non-collecting */ static inline void gc_list_clear_collecting(PyGC_Head *collectable) { PyGC_Head *gc; for (gc = GC_NEXT(collectable); gc != collectable; gc = GC_NEXT(gc)) { gc_clear_collecting(gc); } } /* Append objects in a GC list to a Python list. * Return 0 if all OK, < 0 if error (out of memory for list) */ static int append_objects(PyObject *py_list, PyGC_Head *gc_list) { PyGC_Head *gc; for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) { PyObject *op = FROM_GC(gc); if (op != py_list) { if (PyList_Append(py_list, op)) { return -1; /* exception */ } } } return 0; } // Constants for validate_list's flags argument. enum flagstates {collecting_clear_unreachable_clear, collecting_clear_unreachable_set, collecting_set_unreachable_clear, collecting_set_unreachable_set}; #ifdef GC_DEBUG // validate_list checks list consistency. And it works as document // describing when flags are expected to be set / unset. // `head` must be a doubly-linked gc list, although it's fine (expected!) if // the prev and next pointers are "polluted" with flags. // What's checked: // - The `head` pointers are not polluted. // - The objects' PREV_MASK_COLLECTING and NEXT_MASK_UNREACHABLE flags are all // `set or clear, as specified by the 'flags' argument. // - The prev and next pointers are mutually consistent. static void validate_list(PyGC_Head *head, enum flagstates flags) { assert((head->_gc_prev & PREV_MASK_COLLECTING) == 0); assert((head->_gc_next & NEXT_MASK_UNREACHABLE) == 0); uintptr_t prev_value = 0, next_value = 0; switch (flags) { case collecting_clear_unreachable_clear: break; case collecting_set_unreachable_clear: prev_value = PREV_MASK_COLLECTING; break; case collecting_clear_unreachable_set: next_value = NEXT_MASK_UNREACHABLE; break; case collecting_set_unreachable_set: prev_value = PREV_MASK_COLLECTING; next_value = NEXT_MASK_UNREACHABLE; break; default: assert(! "bad internal flags argument"); } PyGC_Head *prev = head; PyGC_Head *gc = GC_NEXT(head); while (gc != head) { PyGC_Head *trueprev = GC_PREV(gc); PyGC_Head *truenext = (PyGC_Head *)(gc->_gc_next & ~NEXT_MASK_UNREACHABLE); assert(truenext != NULL); assert(trueprev == prev); assert((gc->_gc_prev & PREV_MASK_COLLECTING) == prev_value); assert((gc->_gc_next & NEXT_MASK_UNREACHABLE) == next_value); prev = gc; gc = truenext; } assert(prev == GC_PREV(head)); } #else #define validate_list(x, y) do{}while(0) #endif /*** end of list stuff ***/ /* Set all gc_refs = ob_refcnt. After this, gc_refs is > 0 and * PREV_MASK_COLLECTING bit is set for all objects in containers. */ static Py_ssize_t update_refs(PyGC_Head *containers) { PyGC_Head *next; PyGC_Head *gc = GC_NEXT(containers); Py_ssize_t candidates = 0; while (gc != containers) { next = GC_NEXT(gc); PyObject *op = FROM_GC(gc); if (_Py_IsImmortal(op)) { assert(!_Py_IsStaticImmortal(op)); _PyObject_GC_UNTRACK(op); gc = next; continue; } gc_reset_refs(gc, Py_REFCNT(op)); /* Python's cyclic gc should never see an incoming refcount * of 0: if something decref'ed to 0, it should have been * deallocated immediately at that time. * Possible cause (if the assert triggers): a tp_dealloc * routine left a gc-aware object tracked during its teardown * phase, and did something-- or allowed something to happen -- * that called back into Python. gc can trigger then, and may * see the still-tracked dying object. Before this assert * was added, such mistakes went on to allow gc to try to * delete the object again. In a debug build, that caused * a mysterious segfault, when _Py_ForgetReference tried * to remove the object from the doubly-linked list of all * objects a second time. In a release build, an actual * double deallocation occurred, which leads to corruption * of the allocator's internal bookkeeping pointers. That's * so serious that maybe this should be a release-build * check instead of an assert? */ _PyObject_ASSERT(op, gc_get_refs(gc) != 0); gc = next; candidates++; } return candidates; } /* A traversal callback for subtract_refs. */ static int visit_decref(PyObject *op, void *parent) { OBJECT_STAT_INC(object_visits); _PyObject_ASSERT(_PyObject_CAST(parent), !_PyObject_IsFreed(op)); if (_PyObject_IS_GC(op)) { PyGC_Head *gc = AS_GC(op); /* We're only interested in gc_refs for objects in the * generation being collected, which can be recognized * because only they have positive gc_refs. */ if (gc_is_collecting(gc)) { gc_decref(gc); } } return 0; } int _PyGC_VisitStackRef(_PyStackRef *ref, visitproc visit, void *arg) { // This is a bit tricky! We want to ignore stackrefs with embedded // refcounts when computing the incoming references, but otherwise treat // them like normal. assert(!PyStackRef_IsTaggedInt(*ref)); if (!PyStackRef_RefcountOnObject(*ref) && (visit == visit_decref)) { return 0; } Py_VISIT(PyStackRef_AsPyObjectBorrow(*ref)); return 0; } int _PyGC_VisitFrameStack(_PyInterpreterFrame *frame, visitproc visit, void *arg) { _PyStackRef *ref = _PyFrame_GetLocalsArray(frame); /* locals and stack */ for (; ref < frame->stackpointer; ref++) { if (!PyStackRef_IsTaggedInt(*ref)) { _Py_VISIT_STACKREF(*ref); } } return 0; } /* Subtract internal references from gc_refs. After this, gc_refs is >= 0 * for all objects in containers. The ones with gc_refs > 0 are directly * reachable from outside containers, and so can't be collected. */ static void subtract_refs(PyGC_Head *containers) { traverseproc traverse; PyGC_Head *gc = GC_NEXT(containers); for (; gc != containers; gc = GC_NEXT(gc)) { PyObject *op = FROM_GC(gc); traverse = Py_TYPE(op)->tp_traverse; (void) traverse(op, visit_decref, op); } } /* A traversal callback for move_unreachable. */ static int visit_reachable(PyObject *op, void *arg) { PyGC_Head *reachable = arg; OBJECT_STAT_INC(object_visits); if (!_PyObject_IS_GC(op)) { return 0; } PyGC_Head *gc = AS_GC(op); const Py_ssize_t gc_refs = gc_get_refs(gc); // Ignore objects in other generation. // This also skips objects "to the left" of the current position in // move_unreachable's scan of the 'young' list - they've already been // traversed, and no longer have the PREV_MASK_COLLECTING flag. if (! gc_is_collecting(gc)) { return 0; } // It would be a logic error elsewhere if the collecting flag were set on // an untracked object. _PyObject_ASSERT(op, gc->_gc_next != 0); if (gc->_gc_next & NEXT_MASK_UNREACHABLE) { /* This had gc_refs = 0 when move_unreachable got * to it, but turns out it's reachable after all. * Move it back to move_unreachable's 'young' list, * and move_unreachable will eventually get to it * again. */ // Manually unlink gc from unreachable list because the list functions // don't work right in the presence of NEXT_MASK_UNREACHABLE flags. PyGC_Head *prev = GC_PREV(gc); PyGC_Head *next = (PyGC_Head*)(gc->_gc_next & ~NEXT_MASK_UNREACHABLE); _PyObject_ASSERT(FROM_GC(prev), prev->_gc_next & NEXT_MASK_UNREACHABLE); _PyObject_ASSERT(FROM_GC(next), next->_gc_next & NEXT_MASK_UNREACHABLE); prev->_gc_next = gc->_gc_next; // copy NEXT_MASK_UNREACHABLE _PyGCHead_SET_PREV(next, prev); gc_list_append(gc, reachable); gc_set_refs(gc, 1); } else if (gc_refs == 0) { /* This is in move_unreachable's 'young' list, but * the traversal hasn't yet gotten to it. All * we need to do is tell move_unreachable that it's * reachable. */ gc_set_refs(gc, 1); } /* Else there's nothing to do. * If gc_refs > 0, it must be in move_unreachable's 'young' * list, and move_unreachable will eventually get to it. */ else { _PyObject_ASSERT_WITH_MSG(op, gc_refs > 0, "refcount is too small"); } return 0; } /* Move the unreachable objects from young to unreachable. After this, * all objects in young don't have PREV_MASK_COLLECTING flag and * unreachable have the flag. * All objects in young after this are directly or indirectly reachable * from outside the original young; and all objects in unreachable are * not. * * This function restores _gc_prev pointer. young and unreachable are * doubly linked list after this function. * But _gc_next in unreachable list has NEXT_MASK_UNREACHABLE flag. * So we can not gc_list_* functions for unreachable until we remove the flag. */ static Py_ssize_t move_unreachable(PyGC_Head *young, PyGC_Head *unreachable) { Py_ssize_t reachable = 0; // previous elem in the young list, used for restore gc_prev. PyGC_Head *prev = young; PyGC_Head *gc = GC_NEXT(young); /* Invariants: all objects "to the left" of us in young are reachable * (directly or indirectly) from outside the young list as it was at entry. * * All other objects from the original young "to the left" of us are in * unreachable now, and have NEXT_MASK_UNREACHABLE. All objects to the * left of us in 'young' now have been scanned, and no objects here * or to the right have been scanned yet. */ while (gc != young) { if (gc_get_refs(gc)) { /* gc is definitely reachable from outside the * original 'young'. Mark it as such, and traverse * its pointers to find any other objects that may * be directly reachable from it. Note that the * call to tp_traverse may append objects to young, * so we have to wait until it returns to determine * the next object to visit. */ PyObject *op = FROM_GC(gc); traverseproc traverse = Py_TYPE(op)->tp_traverse; _PyObject_ASSERT_WITH_MSG(op, gc_get_refs(gc) > 0, "refcount is too small"); // NOTE: visit_reachable may change gc->_gc_next when // young->_gc_prev == gc. Don't do gc = GC_NEXT(gc) before! (void) traverse(op, visit_reachable, (void *)young); // relink gc_prev to prev element. _PyGCHead_SET_PREV(gc, prev); // gc is not COLLECTING state after here. gc_clear_collecting(gc); if (PyTuple_CheckExact(op)) { // Untracking gc overwrites the gc_refs of the next object. PyGC_Head *next = GC_NEXT(gc); uintptr_t next_refs = next->_gc_prev; _PyTuple_MaybeUntrack(op); next->_gc_prev = next_refs; if (gc->_gc_next != 0) { prev = gc; reachable++; } } else { prev = gc; reachable++; } } else { /* This *may* be unreachable. To make progress, * assume it is. gc isn't directly reachable from * any object we've already traversed, but may be * reachable from an object we haven't gotten to yet. * visit_reachable will eventually move gc back into * young if that's so, and we'll see it again. */ // Move gc to unreachable. // No need to gc->next->prev = prev because it is single linked. prev->_gc_next = gc->_gc_next; // We can't use gc_list_append() here because we use // NEXT_MASK_UNREACHABLE here. PyGC_Head *last = GC_PREV(unreachable); // NOTE: Since all objects in unreachable set has // NEXT_MASK_UNREACHABLE flag, we set it unconditionally. // But this may pollute the unreachable list head's 'next' pointer // too. That's semantically senseless but expedient here - the // damage is repaired when this function ends. last->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)gc); _PyGCHead_SET_PREV(gc, last); gc->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)unreachable); unreachable->_gc_prev = (uintptr_t)gc; } gc = (PyGC_Head*)prev->_gc_next; } // young->_gc_prev must be last element remained in the list. young->_gc_prev = (uintptr_t)prev; // don't let the pollution of the list head's next pointer leak unreachable->_gc_next &= ~NEXT_MASK_UNREACHABLE; return reachable; } /* Return true if object has a pre-PEP 442 finalization method. */ static int has_legacy_finalizer(PyObject *op) { return Py_TYPE(op)->tp_del != NULL; } /* Move the objects in unreachable with tp_del slots into `finalizers`. * * This function also removes NEXT_MASK_UNREACHABLE flag * from _gc_next in unreachable. */ static void move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers) { PyGC_Head *gc, *next; _PyObject_ASSERT( FROM_GC(unreachable), (unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0); /* March over unreachable. Move objects with finalizers into * `finalizers`. */ for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) { PyObject *op = FROM_GC(gc); _PyObject_ASSERT(op, gc->_gc_next & NEXT_MASK_UNREACHABLE); gc->_gc_next &= ~NEXT_MASK_UNREACHABLE; next = (PyGC_Head*)gc->_gc_next; if (has_legacy_finalizer(op)) { gc_clear_collecting(gc); gc_list_move(gc, finalizers); } } } static inline void clear_unreachable_mask(PyGC_Head *unreachable) { /* Check that the list head does not have the unreachable bit set */ _PyObject_ASSERT( FROM_GC(unreachable), ((uintptr_t)unreachable & NEXT_MASK_UNREACHABLE) == 0); _PyObject_ASSERT( FROM_GC(unreachable), (unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0); PyGC_Head *gc, *next; for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) { _PyObject_ASSERT((PyObject*)FROM_GC(gc), gc->_gc_next & NEXT_MASK_UNREACHABLE); gc->_gc_next &= ~NEXT_MASK_UNREACHABLE; next = (PyGC_Head*)gc->_gc_next; } validate_list(unreachable, collecting_set_unreachable_clear); } /* A traversal callback for move_legacy_finalizer_reachable. */ static int visit_move(PyObject *op, void *arg) { PyGC_Head *tolist = arg; OBJECT_STAT_INC(object_visits); if (_PyObject_IS_GC(op)) { PyGC_Head *gc = AS_GC(op); if (gc_is_collecting(gc)) { gc_list_move(gc, tolist); gc_clear_collecting(gc); } } return 0; } /* Move objects that are reachable from finalizers, from the unreachable set * into finalizers set. */ static void move_legacy_finalizer_reachable(PyGC_Head *finalizers) { traverseproc traverse; PyGC_Head *gc = GC_NEXT(finalizers); for (; gc != finalizers; gc = GC_NEXT(gc)) { /* Note that the finalizers list may grow during this. */ traverse = Py_TYPE(FROM_GC(gc))->tp_traverse; (void) traverse(FROM_GC(gc), visit_move, (void *)finalizers); } } /* Handle weakref callbacks. Note that it's possible for such weakrefs to be * outside the unreachable set -- indeed, those are precisely the weakrefs * whose callbacks must be invoked. See gc_weakref.txt for overview & some * details. * * The clearing of weakrefs is suble and must be done carefully, as there was * previous bugs related to this. First, weakrefs to the unreachable set of * objects must be cleared before we start calling `tp_clear`. If we don't, * those weakrefs can reveal unreachable objects to Python-level code and that * is not safe. Many objects are not usable after `tp_clear` has been called * and could even cause crashes if accessed (see bpo-38006 and gh-91636 as * example bugs). * * Weakrefs with callbacks always need to be cleared before executing the * callback. That's because sometimes a callback will call the ref object, * to check if the reference is actually dead (KeyedRef does this, for * example). We want to indicate that it is dead, even though it is possible * a finalizer might resurrect it. Clearing also prevents the callback from * executing more than once. * * Since Python 2.3, all weakrefs to cyclic garbage have been cleared *before* * calling finalizers. However, since tp_subclasses started being necessary * to invalidate caches (e.g. by PyType_Modified), that clearing has created * a bug. If the weakref to the subclass is cleared before a finalizer is * called, the cache may not be correctly invalidated. That can lead to * segfaults since the caches can refer to deallocated objects (GH-135552 * is an example). Now, we delay the clear of weakrefs without callbacks * until *after* finalizers have been executed. That means weakrefs without * callbacks are still usable while finalizers are being executed. * * The weakrefs that are inside the unreachable set must also be cleared. * The reason this is required is not immediately obvious. If the weakref * refers to an object outside of the unreachable set, that object might go * away when we start clearing objects. Normally, the object should also be * part of the unreachable set but that's not true in the case of incomplete * or missing `tp_traverse` methods. When that object goes away, the callback * for weakref can be executed and that could reveal unreachable objects to * Python-level code. See bpo-38006 as an example bug. */ static int handle_weakref_callbacks(PyGC_Head *unreachable, PyGC_Head *old, bool *needs_resurrection_check) { PyGC_Head *gc; PyGC_Head wrcb_to_call; /* weakrefs with callbacks to call */ PyGC_Head *next; int num_freed = 0; gc_list_init(&wrcb_to_call); /* Find all weakrefs with callbacks and move into `wrcb_to_call` if the * callback needs to be invoked. We make another pass over wrcb_to_call, * invoking callbacks, after this pass completes. */ for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) { PyWeakReference **wrlist; PyObject *op = FROM_GC(gc); next = GC_NEXT(gc); if (! _PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) { continue; } /* It supports weakrefs. Does it have any? * * This is never triggered for static types so we can avoid the * (slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR(). */ wrlist = _PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(op); /* `op` may have some weakrefs. March over the list and move the * weakrefs with callbacks that must be called into wrcb_to_call. */ PyWeakReference *next_wr; for (PyWeakReference *wr = *wrlist; wr != NULL; wr = next_wr) { // Get the next list element to get iterator progress if we omit // clearing of the weakref (because _PyWeakref_ClearRef changes // next pointer in the wrlist). next_wr = wr->wr_next; if (wr->wr_callback == NULL) { /* no callback */ continue; } // Clear the weakref. See the comments above this function for // reasons why we need to clear weakrefs that have callbacks. // Note that _PyWeakref_ClearRef clears the weakref but leaves the // callback pointer intact. Obscure: it also changes *wrlist. _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op); _PyWeakref_ClearRef(wr); _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None); /* Headache time. `op` is going away, and is weakly referenced by * `wr`, which has a callback. Should the callback be invoked? If wr * is also trash, no: * * 1. There's no need to call it. The object and the weakref are * both going away, so it's legitimate to pretend the weakref is * going away first. The user has to ensure a weakref outlives its * referent if they want a guarantee that the wr callback will get * invoked. * * 2. It may be catastrophic to call it. If the callback is also in * cyclic trash (CT), then although the CT is unreachable from * outside the current generation, CT may be reachable from the * callback. Then the callback could resurrect insane objects. * * Since the callback is never needed and may be unsafe in this * case, wr is simply left in the unreachable set. Note that * clear_weakrefs() will ensure its callback will not trigger * inside delete_garbage(). * * OTOH, if wr isn't part of CT, we should invoke the callback: the * weakref outlived the trash. Note that since wr isn't CT in this * case, its callback can't be CT either -- wr acted as an external * root to this generation, and therefore its callback did too. So * nothing in CT is reachable from the callback either, so it's hard * to imagine how calling it later could create a problem for us. wr * is moved to wrcb_to_call in this case. */ if (gc_is_collecting(AS_GC((PyObject *)wr))) { continue; } /* Create a new reference so that wr can't go away * before we can process it again. */ Py_INCREF(wr); /* Move wr to wrcb_to_call, for the next pass. */ PyGC_Head *wrasgc = AS_GC((PyObject *)wr); // wrasgc is reachable, but next isn't, so they can't be the same _PyObject_ASSERT((PyObject *)wr, wrasgc != next); gc_list_move(wrasgc, &wrcb_to_call); } } /* Invoke the callbacks we decided to honor. It's safe to invoke them * because they can't reference unreachable objects. */ if (!*needs_resurrection_check) { *needs_resurrection_check = !gc_list_is_empty(&wrcb_to_call); } while (! gc_list_is_empty(&wrcb_to_call)) { PyObject *temp; PyObject *callback; gc = (PyGC_Head*)wrcb_to_call._gc_next; PyObject *op = FROM_GC(gc); _PyObject_ASSERT(op, PyWeakref_Check(op)); PyWeakReference *wr = (PyWeakReference *)op; callback = wr->wr_callback; _PyObject_ASSERT(op, callback != NULL); /* copy-paste of weakrefobject.c's handle_callback() */ temp = PyObject_CallOneArg(callback, (PyObject *)wr); if (temp == NULL) { PyErr_FormatUnraisable("Exception ignored on " "calling weakref callback %R", callback); } else { Py_DECREF(temp); } /* Give up the reference we created in the first pass. When * op's refcount hits 0 (which it may or may not do right now), * op's tp_dealloc will decref op->wr_callback too. Note * that the refcount probably will hit 0 now, and because this * weakref was reachable to begin with, gc didn't already * add it to its count of freed objects. Example: a reachable * weak value dict maps some key to this reachable weakref. * The callback removes this key->weakref mapping from the * dict, leaving no other references to the weakref (excepting * ours). */ Py_DECREF(op); if (wrcb_to_call._gc_next == (uintptr_t)gc) { /* object is still alive -- move it */ gc_list_move(gc, old); } else { ++num_freed; } } return num_freed; } /* Clear all weakrefs to unreachable objects. When this returns, no object in * `unreachable` is weakly referenced anymore. See the comments above * handle_weakref_callbacks() for why these weakrefs need to be cleared. */ static void clear_weakrefs(PyGC_Head *unreachable) { PyGC_Head *gc; PyGC_Head *next; for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) { PyWeakReference **wrlist; PyObject *op = FROM_GC(gc); next = GC_NEXT(gc); if (PyWeakref_Check(op)) { /* A weakref inside the unreachable set is always cleared. See * the comments above handle_weakref_callbacks() for why these * must be cleared. */ _PyWeakref_ClearRef((PyWeakReference *)op); } if (! _PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) { continue; } /* It supports weakrefs. Does it have any? * * This is never triggered for static types so we can avoid the * (slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR(). */ wrlist = _PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(op); /* `op` may have some weakrefs. March over the list, clear * all the weakrefs. */ for (PyWeakReference *wr = *wrlist; wr != NULL; wr = *wrlist) { /* _PyWeakref_ClearRef clears the weakref but leaves * the callback pointer intact. Obscure: it also * changes *wrlist. */ _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op); _PyWeakref_ClearRef(wr); _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None); } } } /* "gc: " + " <" + " 0x" + 16 hex digits + ">\n" + NUL, excluding msg and * tp_name */ |
| You are visitor 01402520 — GeezerVison does not store the pages you look up. Best viewed with Netscape Navigator 4.7 or Internet Explorer 6 at 800×600. No cookies required. |