libguile/guardians.c

   1 /*      Copyright (C) 1998, 1999, 2000 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 /* Software engineering face-lift by Greg J. Badros, 11-Dec-1999,
  43    gjb@cs.washington.edu, http://www.cs.washington.edu/homes/gjb */
  44
  45 \f
  46
  47 /* This is an implementation of guardians as described in
  48  * R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
  49  * a Generation-Based Garbage Collector" ACM SIGPLAN Conference on
  50  * Programming Language Design and Implementation, June 1993
  51  * ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/guardians.ps.gz
  52  *
  53  * By this point, the semantics are actually quite different from
  54  * those described in the abovementioned paper.  The semantic changes
  55  * are there to improve safety and intuitiveness.  The interface is
  56  * still (mostly) the one described by the paper, however.
  57  *
  58  * Original design:         Mikael Djurfeldt
  59  * Original implementation: Michael Livshin
  60  * Hacked on since by:      everybody
  61  */
  62
  63
  64 #include "libguile/_scm.h"
  65 #include "libguile/ports.h"
  66 #include "libguile/print.h"
  67 #include "libguile/smob.h"
  68 #include "libguile/validate.h"
  69 #include "libguile/root.h"
  70 #include "libguile/hashtab.h"
  71 #include "libguile/weaks.h"
  72
  73 #include "libguile/guardians.h"
  74
  75
  76 /* The live and zombies FIFOs are implemented as tconcs as described
  77    in Dybvig's paper.  This decouples addition and removal of elements
  78    so that no synchronization between these needs to take place.
  79 */
  80
  81 typedef struct tconc_t
  82 {
  83   SCM head;
  84   SCM tail;
  85 } tconc_t;
  86
  87 #define TCONC_EMPTYP(tc) (SCM_EQ_P ((tc).head, (tc).tail))
  88
  89 #define TCONC_IN(tc, obj, pair) \
  90 do { \
  91   SCM_SETCAR ((tc).tail, obj); \
  92   SCM_SETCAR (pair, SCM_BOOL_F); \
  93   SCM_SETCDR (pair, SCM_EOL); \
  94   SCM_SETCDR ((tc).tail, pair); \
  95   (tc).tail = pair; \
  96 } while (0)
  97
  98 #define TCONC_OUT(tc, res) \
  99 do { \
 100   (res) = SCM_CAR ((tc).head); \
 101   (tc).head = SCM_CDR ((tc).head); \
 102 } while (0)
 103
 104
 105 static scm_bits_t tc16_guardian;
 106
 107 typedef struct guardian_t
 108 {
 109   tconc_t live;
 110   tconc_t zombies;
 111   struct guardian_t *next;
 112   int greedy_p;
 113   int listed_p;
 114 } guardian_t;
 115
 116 #define GUARDIAN_P(x) SCM_SMOB_PREDICATE(tc16_guardian, x)
 117 #define GUARDIAN(x) ((guardian_t *) SCM_CELL_WORD_1 (x))
 118 #define GUARDIAN_LIVE(x) (GUARDIAN (x)->live)
 119 #define GUARDIAN_ZOMBIES(x) (GUARDIAN (x)->zombies)
 120 #define GUARDIAN_NEXT(x) (GUARDIAN (x)->next)
 121 #define GUARDIAN_GREEDY_P(x) (GUARDIAN (x)->greedy_p)
 122 #define GUARDIAN_LISTED_P(x) (GUARDIAN (x)->listed_p)
 123
 124
 125 /* during the gc mark phase, live guardians are linked into the lists
 126    here. */
 127 static guardian_t *greedy_guardians = NULL;
 128 static guardian_t *sharing_guardians = NULL;
 129
 130 static SCM greedily_guarded_whash = SCM_EOL;
 131
 132 /* this is the list of guarded objects that are parts of cycles.  we
 133    don't know in which order to return them from guardians, so we just
 134    unguard them and whine about it in after-gc-hook */
 135 static SCM self_centered_zombies = SCM_EOL;
 136
 137
 138 static void
 139 add_to_live_list (SCM g)
 140 {
 141   if (GUARDIAN_LISTED_P (g))
 142     return;
 143
 144   if (GUARDIAN_GREEDY_P (g))
 145     {
 146       GUARDIAN_NEXT (g) = greedy_guardians;
 147       greedy_guardians = GUARDIAN (g);
 148     }
 149   else
 150     {
 151       GUARDIAN_NEXT (g) = sharing_guardians;
 152       sharing_guardians = GUARDIAN (g);
 153     }
 154
 155   GUARDIAN_LISTED_P (g) = 1;
 156 }
 157
 158 /* mark a guardian by adding it to the live guardian list.  */
 159 static SCM
 160 guardian_mark (SCM ptr)
 161 {
 162   add_to_live_list (ptr);
 163
 164   /* the objects protected by the guardian are not marked here: that
 165      would prevent them from ever getting collected.  instead marking
 166      is done at the end of the mark phase by guardian_zombify.  */
 167   return SCM_BOOL_F;
 168 }
 169
 170
 171 static scm_sizet
 172 guardian_free (SCM ptr)
 173 {
 174   scm_must_free (GUARDIAN (ptr));
 175   return sizeof (guardian_t);
 176 }
 177
 178
 179 static int
 180 guardian_print (SCM g, SCM port, scm_print_state *pstate)
 181 {
 182   scm_puts ("#<", port);
 183   if (GUARDIAN_GREEDY_P (g))
 184     scm_puts ("greedy ", port);
 185   else
 186     scm_puts ("sharing ", port);
 187   scm_puts ("guardian (reachable: ", port);
 188   scm_display (scm_length (SCM_CDR (GUARDIAN_LIVE (g).head)), port);
 189   scm_puts (" unreachable: ", port);
 190   scm_display (scm_length (SCM_CDR (GUARDIAN_ZOMBIES (g).head)), port);
 191   scm_puts (")>", port);
 192
 193   return 1;
 194 }
 195
 196
 197 /* This is the Scheme entry point for each guardian: If arg is an object, it's
 198  * added to the guardian's live list.  If arg is unbound, the next available
 199  * zombified object (or #f if none) is returned.
 200  */
 201 static SCM
 202 guardian_apply (SCM guardian, SCM arg)
 203 {
 204   if (!SCM_UNBNDP (arg))
 205     {
 206       scm_guard (guardian, arg);
 207       return SCM_UNSPECIFIED;
 208     }
 209   else
 210     return scm_get_one_zombie (guardian);
 211 }
 212
 213
 214 void
 215 scm_guard (SCM guardian, SCM obj)
 216 {
 217   if (!SCM_IMP (obj))
 218     {
 219       SCM z;
 220
 221       SCM_NEWCELL (z);
 222
 223       /* This critical section barrier will be replaced by a mutex. */
 224       SCM_DEFER_INTS;
 225
 226       if (GUARDIAN_GREEDY_P (guardian))
 227         {
 228           if (SCM_NFALSEP (scm_hashq_get_handle
 229                            (greedily_guarded_whash, obj)))
 230             {
 231               SCM_ALLOW_INTS;
 232               scm_misc_error ("guard",
 233                               "object       is already greedily guarded", obj);
 234             }
 235           else
 236             scm_hashq_create_handle_x (greedily_guarded_whash,
 237                                        obj, guardian);
 238         }
 239
 240       TCONC_IN (GUARDIAN_LIVE (guardian), obj, z);
 241
 242       SCM_ALLOW_INTS;
 243     }
 244
 245 }
 246
 247
 248 SCM
 249 scm_get_one_zombie (SCM guardian)
 250 {
 251   SCM res = SCM_BOOL_F;
 252
 253   /* This critical section barrier will be replaced by a mutex. */
 254   SCM_DEFER_INTS;
 255
 256   if (!TCONC_EMPTYP (GUARDIAN_ZOMBIES (guardian)))
 257     TCONC_OUT (GUARDIAN_ZOMBIES (guardian), res);
 258
 259   if (SCM_NFALSEP (res)
 260       && GUARDIAN_GREEDY_P (guardian)
 261       && SCM_NFALSEP (scm_hashq_get_handle
 262                       (greedily_guarded_whash, res)))
 263     scm_hashq_remove_x (greedily_guarded_whash, res);
 264
 265   SCM_ALLOW_INTS;
 266
 267   return res;
 268 }
 269
 270
 271 SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 1, 0,
 272             (SCM greedy_p),
 273             "Create a new guardian.\n"
 274             "A guardian protects a set of objects from garbage collection,\n"
 275             "allowing a program to apply cleanup or other actions.\n\n"
 276
 277             "make-guardian returns a procedure representing the guardian.\n"
 278             "Calling the guardian procedure with an argument adds the\n"
 279             "argument to the guardian's set of protected objects.\n"
 280             "Calling the guardian procedure without an argument returns\n"
 281             "one of the protected objects which are ready for garbage\n"
 282             "collection or @code{#f} if no such object is available.\n"
 283             "Objects which are returned in this way are removed from\n"
 284             "the guardian.\n\n"
 285
 286             "make-guardian takes one optional argument that says whether the\n"
 287             "new guardian should be greedy or not.  if there is any chance\n"
 288             "that any object protected by the guardian may be resurrected,\n"
 289             "then make the guardian greedy (this is the default).\n\n"
 290
 291             "See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993)\n"
 292             "\"Guardians in a Generation-Based Garbage Collector\".\n"
 293             "ACM SIGPLAN Conference on Programming Language Design\n"
 294             "and Implementation, June 1993.\n\n"
 295
 296             "(the semantics are slightly different at this point, but the\n"
 297             "paper still (mostly) accurately describes the interface).")
 298 #define FUNC_NAME s_scm_make_guardian
 299 {
 300   guardian_t *g = SCM_MUST_MALLOC_TYPE (guardian_t);
 301   SCM z1 = scm_cons (SCM_BOOL_F, SCM_EOL);
 302   SCM z2 = scm_cons (SCM_BOOL_F, SCM_EOL);
 303   SCM z;
 304
 305   /* A tconc starts out with one tail pair. */
 306   g->live.head = g->live.tail = z1;
 307   g->zombies.head = g->zombies.tail = z2;
 308   g->listed_p = 0;
 309
 310   if (SCM_UNBNDP (greedy_p))
 311     g->greedy_p = 1;
 312   else
 313     g->greedy_p = SCM_NFALSEP (greedy_p);
 314
 315   SCM_NEWSMOB (z, tc16_guardian, g);
 316
 317   return z;
 318 }
 319 #undef FUNC_NAME
 320
 321
 322 /* called before gc mark phase begins to initialise the live guardian list. */
 323 static void *
 324 guardian_gc_init (void *dummy1, void *dummy2, void *dummy3)
 325 {
 326   greedy_guardians = sharing_guardians = NULL;
 327
 328   return 0;
 329 }
 330
 331 static void
 332 mark_dependencies_in_tconc (tconc_t *tc)
 333 {
 334   SCM pair, next_pair;
 335   SCM *prev_ptr;
 336
 337   /* scan the list for unmarked objects, and mark their
 338      dependencies */
 339   for (pair = tc->head, prev_ptr = &tc->head;
 340        ! SCM_EQ_P (pair, tc->tail);
 341        pair = next_pair)
 342     {
 343       SCM obj = SCM_CAR (pair);
 344       next_pair = SCM_CDR (pair);
 345
 346       if (! SCM_MARKEDP (obj))
 347         {
 348           /* a candidate for finalizing */
 349           scm_gc_mark_dependencies (obj);
 350
 351           if (SCM_MARKEDP (obj))
 352             {
 353               /* uh oh.  a cycle.  transfer this object (the
 354                  spine cell, to be exact) to
 355                  self_centered_zombies, so we'll be able to
 356                  complain about it later. */
 357               *prev_ptr = next_pair;
 358               SCM_SETGCMARK (pair);
 359               SCM_SETCDR (pair, SCM_CDR (self_centered_zombies));
 360               SCM_SETCDR (self_centered_zombies, pair);
 361             }
 362           else
 363             {
 364               /* see if this is a guardian.  if yes, list it (but don't
 365                  mark it yet). */
 366               if (GUARDIAN_P (obj))
 367                 add_to_live_list (obj);
 368
 369               prev_ptr = SCM_CDRLOC (pair);
 370             }
 371         }
 372     }
 373 }
 374
 375 static void
 376 mark_dependencies (guardian_t *g)
 377 {
 378   mark_dependencies_in_tconc (&g->zombies);
 379   mark_dependencies_in_tconc (&g->live);
 380 }
 381
 382 static void
 383 mark_and_zombify (guardian_t *g)
 384 {
 385   SCM tconc_tail = g->live.tail;
 386   SCM *prev_ptr = &g->live.head;
 387   SCM pair = g->live.head;
 388
 389   while (! SCM_EQ_P (pair, tconc_tail))
 390     {
 391       SCM next_pair = SCM_CDR (pair);
 392
 393       if (SCM_NMARKEDP (SCM_CAR (pair)))
 394         {
 395           /* got you, zombie! */
 396
 397           /* out of the live list! */
 398           *prev_ptr = next_pair;
 399
 400           if (g->greedy_p)
 401             /* if the guardian is greedy, mark this zombie now.  this
 402                way it won't be zombified again this time around. */
 403             SCM_SETGCMARK (SCM_CAR (pair));
 404
 405           /* into the zombie list! */
 406           TCONC_IN (g->zombies, SCM_CAR (pair), pair);
 407         }
 408       else
 409         prev_ptr = SCM_CDRLOC (pair);
 410
 411       pair = next_pair;
 412     }
 413
 414   /* Mark the cells of the live list (yes, the cells in the list, we
 415      don't care about objects pointed to by the list cars, since we
 416      know they are already marked).  */
 417   for (pair = g->live.head; !SCM_NULLP (pair); pair = SCM_CDR (pair))
 418     SCM_SETGCMARK (pair);
 419 }
 420
 421
 422 /* this is called by the garbage collector between the mark and sweep
 423    phases.  for each marked guardian, it moves any unmarked object in
 424    its live list (tconc) to its zombie list (tconc).  */
 425 static void *
 426 guardian_zombify (void *dummy1, void *dummy2, void *dummy3)
 427 {
 428   guardian_t *last_greedy_guardian = NULL;
 429   guardian_t *last_sharing_guardian = NULL;
 430   guardian_t *first_greedy_guardian = NULL;
 431   guardian_t *first_sharing_guardian = NULL;
 432   guardian_t *g;
 433
 434   /* First, find all newly unreachable objects and mark their
 435      dependencies.
 436
 437      Note that new guardians may be stuck on the end of the live
 438      guardian lists as we run this loop, since guardians might be
 439      guarded too.  When we mark a guarded guardian, its mark function
 440      sticks in the appropriate live guardian list.  The loop
 441      terminates when no new guardians are found. */
 442
 443   do {
 444     first_greedy_guardian = greedy_guardians;
 445     first_sharing_guardian = sharing_guardians;
 446
 447     for (g = greedy_guardians; g != last_greedy_guardian;
 448          g = g->next)
 449       mark_dependencies (g);
 450     for (g = sharing_guardians; g != last_sharing_guardian;
 451          g = g->next)
 452       mark_dependencies (g);
 453
 454     last_greedy_guardian = first_greedy_guardian;
 455     last_sharing_guardian = first_sharing_guardian;
 456   } while (first_greedy_guardian != greedy_guardians
 457            || first_sharing_guardian != sharing_guardians);
 458
 459   /* now, scan all the guardians that are currently known to be live
 460      and move their unmarked objects to zombie lists. */
 461
 462   for (g = greedy_guardians; g; g = g->next)
 463     {
 464       mark_and_zombify (g);
 465       g->listed_p = 0;
 466     }
 467   for (g = sharing_guardians; g; g = g->next)
 468     {
 469       mark_and_zombify (g);
 470       g->listed_p = 0;
 471     }
 472
 473   /* Preserve the zombies in their undead state, by marking to prevent
 474      collection. */
 475   for (g = greedy_guardians; g; g = g->next)
 476     scm_gc_mark (g->zombies.head);
 477   for (g = sharing_guardians; g; g = g->next)
 478     scm_gc_mark (g->zombies.head);
 479
 480   return 0;
 481 }
 482
 483 static void *
 484 whine_about_self_centered_zombies (void *dummy1, void *dummy2, void *dummy3)
 485 {
 486   if (! SCM_NULLP (SCM_CDR (self_centered_zombies)))
 487     {
 488       SCM pair;
 489
 490       scm_puts ("** WARNING: the following guarded objects were unguarded due to cycles:",
 491                 scm_cur_errp);
 492       scm_newline (scm_cur_errp);
 493       for (pair = SCM_CDR (self_centered_zombies);
 494            ! SCM_NULLP (pair); pair = SCM_CDR (pair))
 495         {
 496           scm_display (SCM_CAR (pair), scm_cur_errp);
 497           scm_newline (scm_cur_errp);
 498         }
 499
 500       SCM_SETCDR (self_centered_zombies, SCM_EOL);
 501     }
 502
 503   return 0;
 504 }
 505
 506 void
 507 scm_init_guardians ()
 508 {
 509   tc16_guardian = scm_make_smob_type ("guardian", 0);
 510   scm_set_smob_mark (tc16_guardian, guardian_mark);
 511   scm_set_smob_free (tc16_guardian, guardian_free);
 512   scm_set_smob_print (tc16_guardian, guardian_print);
 513   scm_set_smob_apply (tc16_guardian, guardian_apply, 0, 1, 0);
 514
 515   scm_c_hook_add (&scm_before_mark_c_hook, guardian_gc_init, 0, 0);
 516   scm_c_hook_add (&scm_before_sweep_c_hook, guardian_zombify, 0, 0);
 517
 518   self_centered_zombies =
 519     scm_permanent_object (scm_cons (SCM_UNDEFINED, SCM_EOL));
 520   scm_c_hook_add (&scm_after_gc_c_hook,
 521                   whine_about_self_centered_zombies, 0, 0);
 522
 523   greedily_guarded_whash =
 524     scm_permanent_object (scm_make_doubly_weak_hash_table (SCM_MAKINUM (31)));
 525
 526 #ifndef SCM_MAGIC_SNARFER
 527 #include "libguile/guardians.x"
 528 #endif
 529 }
 530
 531 /*
 532   Local Variables:
 533   c-file-style: "gnu"
 534   End:
 535 */