libguile/guardians.c

   1 /* Copyright (C) 1998,1999,2000,2001 Free Software Foundation, Inc.
   2  *
   3  * This program is free software; you can redistribute it and/or modify
   4  * it under the terms of the GNU General Public License as published by
   5  * the Free Software Foundation; either version 2, or (at your option)
   6  * any later version.
   7  *
   8  * This program is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  11  * GNU General Public License for more details.
  12  *
  13  * You should have received a copy of the GNU General Public License
  14  * along with this software; see the file COPYING.  If not, write to
  15  * the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
  16  * Boston, MA 02111-1307 USA
  17  *
  18  * As a special exception, the Free Software Foundation gives permission
  19  * for additional uses of the text contained in its release of GUILE.
  20  *
  21  * The exception is that, if you link the GUILE library with other files
  22  * to produce an executable, this does not by itself cause the
  23  * resulting executable to be covered by the GNU General Public License.
  24  * Your use of that executable is in no way restricted on account of
  25  * linking the GUILE library code into it.
  26  *
  27  * This exception does not however invalidate any other reasons why
  28  * the executable file might be covered by the GNU General Public License.
  29  *
  30  * This exception applies only to the code released by the
  31  * Free Software Foundation under the name GUILE.  If you copy
  32  * code from other Free Software Foundation releases into a copy of
  33  * GUILE, as the General Public License permits, the exception does
  34  * not apply to the code that you add in this way.  To avoid misleading
  35  * anyone as to the status of such modified files, you must delete
  36  * this exception notice from them.
  37  *
  38  * If you write modifications of your own for GUILE, it is your choice
  39  * whether to permit this exception to apply to your modifications.
  40  * If you do not wish that, delete this exception notice.  */
  41
  42 \f
  43
  44 /* This is an implementation of guardians as described in
  45  * R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
  46  * a Generation-Based Garbage Collector" ACM SIGPLAN Conference on
  47  * Programming Language Design and Implementation, June 1993
  48  * ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/guardians.ps.gz
  49  *
  50  * By this point, the semantics are actually quite different from
  51  * those described in the abovementioned paper.  The semantic changes
  52  * are there to improve safety and intuitiveness.  The interface is
  53  * still (mostly) the one described by the paper, however.
  54  *
  55  * Original design:         Mikael Djurfeldt
  56  * Original implementation: Michael Livshin
  57  * Hacked on since by:      everybody
  58  */
  59
  60
  61 #include "libguile/_scm.h"
  62 #include "libguile/ports.h"
  63 #include "libguile/print.h"
  64 #include "libguile/smob.h"
  65 #include "libguile/validate.h"
  66 #include "libguile/root.h"
  67 #include "libguile/hashtab.h"
  68 #include "libguile/weaks.h"
  69
  70 #include "libguile/guardians.h"
  71
  72
  73 /* The live and zombies FIFOs are implemented as tconcs as described
  74    in Dybvig's paper.  This decouples addition and removal of elements
  75    so that no synchronization between these needs to take place.
  76 */
  77
  78 typedef struct t_tconc
  79 {
  80   SCM head;
  81   SCM tail;
  82 } t_tconc;
  83
  84 #define TCONC_EMPTYP(tc) (SCM_EQ_P ((tc).head, (tc).tail))
  85
  86 #define TCONC_IN(tc, obj, pair) \
  87 do { \
  88   SCM_SETCAR ((tc).tail, obj); \
  89   SCM_SET_CELL_WORD_1 (pair, SCM_EOL); \
  90   SCM_SET_CELL_WORD_0 (pair, SCM_BOOL_F); \
  91   SCM_SETCDR ((tc).tail, pair); \
  92   (tc).tail = pair; \
  93 } while (0)
  94
  95 #define TCONC_OUT(tc, res) \
  96 do { \
  97   (res) = SCM_CAR ((tc).head); \
  98   (tc).head = SCM_CDR ((tc).head); \
  99 } while (0)
 100
 101
 102 static scm_t_bits tc16_guardian;
 103
 104 typedef struct t_guardian
 105 {
 106   t_tconc live;
 107   t_tconc zombies;
 108   struct t_guardian *next;
 109   unsigned long flags;
 110 } t_guardian;
 111
 112 #define GUARDIAN_P(x) SCM_SMOB_PREDICATE(tc16_guardian, x)
 113 #define GUARDIAN_DATA(x) ((t_guardian *) SCM_CELL_WORD_1 (x))
 114
 115 #define F_GREEDY 1L
 116 #define F_LISTED (1L << 1)
 117 #define F_DESTROYED (1L << 2)
 118
 119 #define GREEDY_P(x)   (((x)->flags & F_GREEDY) != 0)
 120 #define SET_GREEDY(x) ((x)->flags |= F_GREEDY)
 121
 122 #define LISTED_P(x)   (((x)->flags & F_LISTED) != 0)
 123 #define SET_LISTED(x) ((x)->flags |= F_LISTED)
 124 #define CLR_LISTED(x) ((x)->flags &= ~F_LISTED)
 125
 126 #define DESTROYED_P(x)   (((x)->flags & F_DESTROYED) != 0)
 127 #define SET_DESTROYED(x) ((x)->flags |= F_DESTROYED)
 128
 129 /* during the gc mark phase, live guardians are linked into the lists
 130    here. */
 131 static t_guardian *greedy_guardians = NULL;
 132 static t_guardian *sharing_guardians = NULL;
 133
 134 static SCM greedily_guarded_whash = SCM_EOL;
 135
 136 /* this is the list of guarded objects that are parts of cycles.  we
 137    don't know in which order to return them from guardians, so we just
 138    unguard them and whine about it in after-gc-hook */
 139 static SCM self_centered_zombies = SCM_EOL;
 140
 141
 142 static void
 143 add_to_live_list (t_guardian *g)
 144 {
 145   if (LISTED_P (g))
 146     return;
 147
 148   if (GREEDY_P (g))
 149     {
 150       g->next = greedy_guardians;
 151       greedy_guardians = g;
 152     }
 153   else
 154     {
 155       g->next = sharing_guardians;
 156       sharing_guardians = g;
 157     }
 158
 159   SET_LISTED (g);
 160 }
 161
 162 /* mark a guardian by adding it to the live guardian list.  */
 163 static SCM
 164 guardian_mark (SCM ptr)
 165 {
 166   add_to_live_list (GUARDIAN_DATA (ptr));
 167
 168   /* the objects protected by the guardian are not marked here: that
 169      would prevent them from ever getting collected.  instead marking
 170      is done at the end of the mark phase by guardian_zombify.  */
 171   return SCM_BOOL_F;
 172 }
 173
 174
 175 static size_t
 176 guardian_free (SCM ptr)
 177 {
 178   scm_gc_free (GUARDIAN_DATA (ptr), sizeof (t_guardian), "guardian");
 179   return 0;
 180 }
 181
 182
 183 static int
 184 guardian_print (SCM guardian, SCM port, scm_print_state *pstate SCM_UNUSED)
 185 {
 186   t_guardian *g = GUARDIAN_DATA (guardian);
 187
 188   scm_puts ("#<", port);
 189
 190   if (DESTROYED_P (g))
 191     scm_puts ("destroyed ", port);
 192
 193   if (GREEDY_P (g))
 194     scm_puts ("greedy", port);
 195   else
 196     scm_puts ("sharing", port);
 197
 198   scm_puts (" guardian 0x", port);
 199   scm_intprint ((long) g, 16, port);
 200
 201   if (! DESTROYED_P (g))
 202     {
 203       scm_puts (" (reachable: ", port);
 204       scm_display (scm_length (SCM_CDR (g->live.head)), port);
 205       scm_puts (" unreachable: ", port);
 206       scm_display (scm_length (SCM_CDR (g->zombies.head)), port);
 207       scm_puts (")", port);
 208     }
 209
 210   scm_puts (">", port);
 211
 212   return 1;
 213 }
 214
 215
 216 /* This is the Scheme entry point for each guardian: If OBJ is an
 217  * object, it's added to the guardian's live list.  If OBJ is unbound,
 218  * the next available unreachable object (or #f if none) is returned.
 219  *
 220  * If the second optional argument THROW_P is true (the default), then
 221  * an error is raised if GUARDIAN is greedy and OBJ is already greedily
 222  * guarded.  If THROW_P is false, #f is returned instead of raising the
 223  * error, and #t is returned if everything is fine.
 224  */
 225 static SCM
 226 guardian_apply (SCM guardian, SCM obj, SCM throw_p)
 227 {
 228   if (DESTROYED_P (GUARDIAN_DATA (guardian)))
 229     scm_misc_error ("guard", "attempted use of destroyed guardian: ~A",
 230                     scm_list_1 (guardian));
 231
 232   if (!SCM_UNBNDP (obj))
 233     return scm_guard (guardian, obj,
 234                       (SCM_UNBNDP (throw_p)
 235                        ? 1
 236                        : !SCM_FALSEP (throw_p)));
 237   else
 238     return scm_get_one_zombie (guardian);
 239 }
 240
 241
 242 SCM
 243 scm_guard (SCM guardian, SCM obj, int throw_p)
 244 {
 245   t_guardian *g = GUARDIAN_DATA (guardian);
 246
 247   if (!SCM_IMP (obj))
 248     {
 249       SCM z;
 250
 251       /* This critical section barrier will be replaced by a mutex. */
 252       SCM_DEFER_INTS;
 253
 254       if (GREEDY_P (g))
 255         {
 256           if (!SCM_FALSEP (scm_hashq_get_handle
 257                            (greedily_guarded_whash, obj)))
 258             {
 259               SCM_ALLOW_INTS;
 260
 261               if (throw_p)
 262                 scm_misc_error ("guard",
 263                                 "object is already greedily guarded: ~A",
 264                                 scm_list_1 (obj));
 265               else
 266                 return SCM_BOOL_F;
 267             }
 268           else
 269             scm_hashq_create_handle_x (greedily_guarded_whash,
 270                                        obj, guardian);
 271         }
 272
 273       z = scm_cons (SCM_BOOL_F, SCM_BOOL_F);
 274       TCONC_IN (g->live, obj, z);
 275
 276       SCM_ALLOW_INTS;
 277     }
 278
 279   return throw_p ? SCM_UNSPECIFIED : SCM_BOOL_T;
 280 }
 281
 282
 283 SCM
 284 scm_get_one_zombie (SCM guardian)
 285 {
 286   t_guardian *g = GUARDIAN_DATA (guardian);
 287   SCM res = SCM_BOOL_F;
 288
 289   /* This critical section barrier will be replaced by a mutex. */
 290   SCM_DEFER_INTS;
 291
 292   if (!TCONC_EMPTYP (g->zombies))
 293     TCONC_OUT (g->zombies, res);
 294
 295   if (!SCM_FALSEP (res) && GREEDY_P (g))
 296     scm_hashq_remove_x (greedily_guarded_whash, res);
 297
 298   SCM_ALLOW_INTS;
 299
 300   return res;
 301 }
 302
 303
 304 SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 1, 0,
 305             (SCM greedy_p),
 306             "Create a new guardian.\n"
 307             "A guardian protects a set of objects from garbage collection,\n"
 308             "allowing a program to apply cleanup or other actions.\n\n"
 309
 310             "@code{make-guardian} returns a procedure representing the guardian.\n"
 311             "Calling the guardian procedure with an argument adds the\n"
 312             "argument to the guardian's set of protected objects.\n"
 313             "Calling the guardian procedure without an argument returns\n"
 314             "one of the protected objects which are ready for garbage\n"
 315             "collection, or @code{#f} if no such object is available.\n"
 316             "Objects which are returned in this way are removed from\n"
 317             "the guardian.\n\n"
 318
 319             "@code{make-guardian} takes one optional argument that says whether the\n"
 320             "new guardian should be greedy or sharing.  If there is any chance\n"
 321             "that any object protected by the guardian may be resurrected,\n"
 322             "then you should make the guardian greedy (this is the default).\n\n"
 323
 324             "See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993)\n"
 325             "\"Guardians in a Generation-Based Garbage Collector\".\n"
 326             "ACM SIGPLAN Conference on Programming Language Design\n"
 327             "and Implementation, June 1993.\n\n"
 328
 329             "(the semantics are slightly different at this point, but the\n"
 330             "paper still (mostly) accurately describes the interface).")
 331 #define FUNC_NAME s_scm_make_guardian
 332 {
 333   t_guardian *g = scm_gc_malloc (sizeof (t_guardian), "guardian");
 334   SCM z1 = scm_cons (SCM_BOOL_F, SCM_EOL);
 335   SCM z2 = scm_cons (SCM_BOOL_F, SCM_EOL);
 336   SCM z;
 337
 338   /* A tconc starts out with one tail pair. */
 339   g->live.head = g->live.tail = z1;
 340   g->zombies.head = g->zombies.tail = z2;
 341
 342   g->next = NULL;
 343   g->flags = 0L;
 344
 345   /* [cmm] the UNBNDP check below is redundant but I like it. */
 346   if (SCM_UNBNDP (greedy_p) || !SCM_FALSEP (greedy_p))
 347     SET_GREEDY (g);
 348
 349   SCM_NEWSMOB (z, tc16_guardian, g);
 350
 351   return z;
 352 }
 353 #undef FUNC_NAME
 354
 355
 356 SCM_DEFINE (scm_guardian_destroyed_p, "guardian-destroyed?", 1, 0, 0,
 357             (SCM guardian),
 358             "Return @code{#t} if @var{guardian} has been destroyed, otherwise @code{#f}.")
 359 #define FUNC_NAME s_scm_guardian_destroyed_p
 360 {
 361   SCM res = SCM_BOOL_F;
 362
 363   /* This critical section barrier will be replaced by a mutex. */
 364   SCM_DEFER_INTS;
 365
 366   res = SCM_BOOL (DESTROYED_P (GUARDIAN_DATA (guardian)));
 367
 368   SCM_ALLOW_INTS;
 369
 370   return res;
 371 }
 372 #undef FUNC_NAME
 373
 374 SCM_DEFINE (scm_guardian_greedy_p, "guardian-greedy?", 1, 0, 0,
 375             (SCM guardian),
 376             "Return @code{#t} if @var{guardian} is a greedy guardian, otherwise @code{#f}.")
 377 #define FUNC_NAME s_scm_guardian_greedy_p
 378 {
 379   return SCM_BOOL (GREEDY_P (GUARDIAN_DATA (guardian)));
 380 }
 381 #undef FUNC_NAME
 382
 383 SCM_DEFINE (scm_destroy_guardian_x, "destroy-guardian!", 1, 0, 0,
 384             (SCM guardian),
 385             "Destroys @var{guardian}, by making it impossible to put any more\n"
 386             "objects in it or get any objects from it.  It also unguards any\n"
 387             "objects guarded by @var{guardian}.")
 388 #define FUNC_NAME s_scm_destroy_guardian_x
 389 {
 390   t_guardian *g = GUARDIAN_DATA (guardian);
 391
 392   /* This critical section barrier will be replaced by a mutex. */
 393   SCM_DEFER_INTS;
 394
 395   if (DESTROYED_P (g))
 396     {
 397       SCM_ALLOW_INTS;
 398       SCM_MISC_ERROR ("guardian is already destroyed: ~A",
 399                       scm_list_1 (guardian));
 400     }
 401
 402   if (GREEDY_P (g))
 403     {
 404       /* clear the "greedily guarded" property of the objects */
 405       SCM pair;
 406       for (pair = g->live.head; pair != g->live.tail; pair = SCM_CDR (pair))
 407         scm_hashq_remove_x (greedily_guarded_whash, SCM_CAR (pair));
 408       for (pair = g->zombies.head; pair != g->zombies.tail; pair = SCM_CDR (pair))
 409         scm_hashq_remove_x (greedily_guarded_whash, SCM_CAR (pair));
 410     }
 411
 412   /* empty the lists */
 413   g->live.head = g->live.tail;
 414   g->zombies.head = g->zombies.tail;
 415
 416   SET_DESTROYED (g);
 417
 418   SCM_ALLOW_INTS;
 419
 420   return SCM_UNSPECIFIED;
 421 }
 422 #undef FUNC_NAME
 423
 424 /* called before gc mark phase begins to initialise the live guardian list. */
 425 static void *
 426 guardian_gc_init (void *dummy1 SCM_UNUSED,
 427                   void *dummy2 SCM_UNUSED,
 428                   void *dummy3 SCM_UNUSED)
 429 {
 430   greedy_guardians = sharing_guardians = NULL;
 431
 432   return 0;
 433 }
 434
 435 static void
 436 mark_dependencies_in_tconc (t_tconc *tc)
 437 {
 438   SCM pair, next_pair;
 439   SCM *prev_ptr;
 440
 441   /* scan the list for unmarked objects, and mark their
 442      dependencies */
 443   for (pair = tc->head, prev_ptr = &tc->head;
 444        ! SCM_EQ_P (pair, tc->tail);
 445        pair = next_pair)
 446     {
 447       SCM obj = SCM_CAR (pair);
 448       next_pair = SCM_CDR (pair);
 449
 450       if (! SCM_GC_MARK_P (obj))
 451         {
 452           /* a candidate for finalizing */
 453           scm_gc_mark_dependencies (obj);
 454
 455           if (SCM_GC_MARK_P (obj))
 456             {
 457               /* uh oh.  a cycle.  transfer this object (the
 458                  spine cell, to be exact) to
 459                  self_centered_zombies, so we'll be able to
 460                  complain about it later. */
 461               *prev_ptr = next_pair;
 462               SCM_SET_GC_MARK (pair);
 463               SCM_SETCDR (pair, self_centered_zombies);
 464               self_centered_zombies = pair;
 465             }
 466           else
 467             {
 468               /* see if this is a guardian.  if yes, list it (but don't
 469                  mark it yet). */
 470               if (GUARDIAN_P (obj))
 471                 add_to_live_list (GUARDIAN_DATA (obj));
 472
 473               prev_ptr = SCM_CDRLOC (pair);
 474             }
 475         }
 476     }
 477 }
 478
 479 static void
 480 mark_dependencies (t_guardian *g)
 481 {
 482   mark_dependencies_in_tconc (&g->zombies);
 483   mark_dependencies_in_tconc (&g->live);
 484 }
 485
 486 static void
 487 mark_and_zombify (t_guardian *g)
 488 {
 489   SCM tconc_tail = g->live.tail;
 490   SCM *prev_ptr = &g->live.head;
 491   SCM pair = g->live.head;
 492
 493   while (! SCM_EQ_P (pair, tconc_tail))
 494     {
 495       SCM next_pair = SCM_CDR (pair);
 496
 497       if (!SCM_GC_MARK_P (SCM_CAR (pair)))
 498         {
 499           /* got you, zombie! */
 500
 501           /* out of the live list! */
 502           *prev_ptr = next_pair;
 503
 504           if (GREEDY_P (g))
 505             /* if the guardian is greedy, mark this zombie now.  this
 506                way it won't be zombified again this time around. */
 507             SCM_SET_GC_MARK (SCM_CAR (pair));
 508
 509           /* into the zombie list! */
 510           TCONC_IN (g->zombies, SCM_CAR (pair), pair);
 511         }
 512       else
 513         prev_ptr = SCM_CDRLOC (pair);
 514
 515       pair = next_pair;
 516     }
 517
 518   /* Mark the cells of the live list (yes, the cells in the list, we
 519      don't care about objects pointed to by the list cars, since we
 520      know they are already marked).  */
 521   for (pair = g->live.head; !SCM_NULLP (pair); pair = SCM_CDR (pair))
 522     SCM_SET_GC_MARK (pair);
 523 }
 524
 525
 526 /* this is called by the garbage collector between the mark and sweep
 527    phases.  for each marked guardian, it moves any unmarked object in
 528    its live list (tconc) to its zombie list (tconc).  */
 529 static void *
 530 guardian_zombify (void *dummy1 SCM_UNUSED,
 531                   void *dummy2 SCM_UNUSED,
 532                   void *dummy3 SCM_UNUSED)
 533 {
 534   t_guardian *last_greedy_guardian = NULL;
 535   t_guardian *last_sharing_guardian = NULL;
 536   t_guardian *first_greedy_guardian = NULL;
 537   t_guardian *first_sharing_guardian = NULL;
 538   t_guardian *g;
 539
 540   /* First, find all newly unreachable objects and mark their
 541      dependencies.
 542
 543      Note that new guardians may be stuck on the end of the live
 544      guardian lists as we run this loop, since guardians might be
 545      guarded too.  When we mark a guarded guardian, its mark function
 546      sticks in the appropriate live guardian list.  The loop
 547      terminates when no new guardians are found. */
 548
 549   do {
 550     first_greedy_guardian = greedy_guardians;
 551     first_sharing_guardian = sharing_guardians;
 552
 553     for (g = greedy_guardians; g != last_greedy_guardian;
 554          g = g->next)
 555       mark_dependencies (g);
 556     for (g = sharing_guardians; g != last_sharing_guardian;
 557          g = g->next)
 558       mark_dependencies (g);
 559
 560     last_greedy_guardian = first_greedy_guardian;
 561     last_sharing_guardian = first_sharing_guardian;
 562   } while (first_greedy_guardian != greedy_guardians
 563            || first_sharing_guardian != sharing_guardians);
 564
 565   /* now, scan all the guardians that are currently known to be live
 566      and move their unmarked objects to zombie lists. */
 567
 568   for (g = greedy_guardians; g; g = g->next)
 569     {
 570       mark_and_zombify (g);
 571       CLR_LISTED (g);
 572     }
 573   for (g = sharing_guardians; g; g = g->next)
 574     {
 575       mark_and_zombify (g);
 576       CLR_LISTED (g);
 577     }
 578
 579   /* Preserve the zombies in their undead state, by marking to prevent
 580      collection. */
 581   for (g = greedy_guardians; g; g = g->next)
 582     scm_gc_mark (g->zombies.head);
 583   for (g = sharing_guardians; g; g = g->next)
 584     scm_gc_mark (g->zombies.head);
 585
 586   return 0;
 587 }
 588
 589 static void *
 590 whine_about_self_centered_zombies (void *dummy1 SCM_UNUSED,
 591                                    void *dummy2 SCM_UNUSED,
 592                                    void *dummy3 SCM_UNUSED)
 593 {
 594   if (!SCM_NULLP (self_centered_zombies))
 595     {
 596       SCM pair;
 597
 598       scm_puts ("** WARNING: the following guarded objects were unguarded due to cycles:",
 599                 scm_cur_errp);
 600       scm_newline (scm_cur_errp);
 601       for (pair = self_centered_zombies;
 602            !SCM_NULLP (pair); pair = SCM_CDR (pair))
 603         {
 604           scm_display (SCM_CAR (pair), scm_cur_errp);
 605           scm_newline (scm_cur_errp);
 606         }
 607
 608       self_centered_zombies = SCM_EOL;
 609     }
 610
 611   return 0;
 612 }
 613
 614 void
 615 scm_init_guardians ()
 616 {
 617   tc16_guardian = scm_make_smob_type ("guardian", 0);
 618   scm_set_smob_mark (tc16_guardian, guardian_mark);
 619   scm_set_smob_free (tc16_guardian, guardian_free);
 620   scm_set_smob_print (tc16_guardian, guardian_print);
 621   scm_set_smob_apply (tc16_guardian, guardian_apply, 0, 2, 0);
 622
 623   scm_c_hook_add (&scm_before_mark_c_hook, guardian_gc_init, 0, 0);
 624   scm_c_hook_add (&scm_before_sweep_c_hook, guardian_zombify, 0, 0);
 625
 626   scm_gc_register_root (&self_centered_zombies);
 627   scm_c_hook_add (&scm_after_gc_c_hook,
 628                   whine_about_self_centered_zombies, 0, 0);
 629
 630   greedily_guarded_whash =
 631     scm_permanent_object (scm_make_doubly_weak_hash_table (SCM_MAKINUM (31)));
 632
 633 #include "libguile/guardians.x"
 634 }
 635
 636 /*
 637   Local Variables:
 638   c-file-style: "gnu"
 639   End:
 640 */