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