Stable.c 14.4 KB
Newer Older
1 2
/* -----------------------------------------------------------------------------
 *
3
 * (c) The GHC Team, 1998-2002
4 5 6 7 8
 *
 * Stable names and stable pointers.
 *
 * ---------------------------------------------------------------------------*/

9
#include "PosixSource.h"
10
#include "Rts.h"
Simon Marlow's avatar
Simon Marlow committed
11 12
#include "RtsAPI.h"

13 14
#include "Hash.h"
#include "RtsUtils.h"
Simon Marlow's avatar
Simon Marlow committed
15
#include "Trace.h"
16
#include "Stable.h"
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

/* Comment from ADR's implementation in old RTS:

  This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
  small change in @HpOverflow.lc@) consists of the changes in the
  runtime system required to implement "Stable Pointers". But we're
  getting a bit ahead of ourselves --- what is a stable pointer and what
  is it used for?

  When Haskell calls C, it normally just passes over primitive integers,
  floats, bools, strings, etc.  This doesn't cause any problems at all
  for garbage collection because the act of passing them makes a copy
  from the heap, stack or wherever they are onto the C-world stack.
  However, if we were to pass a heap object such as a (Haskell) @String@
  and a garbage collection occured before we finished using it, we'd run
  into problems since the heap object might have been moved or even
  deleted.

  So, if a C call is able to cause a garbage collection or we want to
  store a pointer to a heap object between C calls, we must be careful
  when passing heap objects. Our solution is to keep a table of all
  objects we've given to the C-world and to make sure that the garbage
  collector collects these objects --- updating the table as required to
  make sure we can still find the object.


  Of course, all this rather begs the question: why would we want to
  pass a boxed value?

  One very good reason is to preserve laziness across the language
  interface. Rather than evaluating an integer or a string because it
  {\em might\/} be required by the C function, we can wait until the C
  function actually wants the value and then force an evaluation.

  Another very good reason (the motivating reason!) is that the C code
  might want to execute an object of sort $IO ()$ for the side-effects
  it will produce. For example, this is used when interfacing to an X
  widgets library to allow a direct implementation of callbacks.


  The @makeStablePointer :: a -> IO (StablePtr a)@ function
  converts a value into a stable pointer.  It is part of the @PrimIO@
  monad, because we want to be sure we don't allocate one twice by
  accident, and then only free one of the copies.

  \begin{verbatim}
  makeStablePtr#  :: a -> State# RealWorld -> (# RealWorld, a #)
  freeStablePtr#  :: StablePtr# a -> State# RealWorld -> State# RealWorld
  deRefStablePtr# :: StablePtr# a -> State# RealWorld -> 
        (# State# RealWorld, a #)
  \end{verbatim}

  There may be additional functions on the C side to allow evaluation,
  application, etc of a stable pointer.

*/

74 75
snEntry *stable_ptr_table = NULL;
static snEntry *stable_ptr_free = NULL;
76

77
static unsigned int SPT_size = 0;
78

79
#ifdef THREADED_RTS
80
Mutex stable_mutex;
81
#endif
82

Simon Marlow's avatar
Simon Marlow committed
83 84
static void enlargeStablePtrTable(void);

85 86 87 88
/* This hash table maps Haskell objects to stable names, so that every
 * call to lookupStableName on a given object will return the same
 * stable name.
 *
89 90 91
 * OLD COMMENTS about reference counting follow.  The reference count
 * in a stable name entry is now just a counter.
 *
92 93 94 95 96 97 98 99 100 101
 * Reference counting
 * ------------------
 * A plain stable name entry has a zero reference count, which means
 * the entry will dissappear when the object it points to is
 * unreachable.  For stable pointers, we need an entry that sticks
 * around and keeps the object it points to alive, so each stable name
 * entry has an associated reference count.
 *
 * A stable pointer has a weighted reference count N attached to it
 * (actually in its upper 5 bits), which represents the weight
102
 * 2^(N-1).  The stable name entry keeps a 32-bit reference count, which
103 104 105 106 107 108 109 110 111 112 113 114 115 116 117
 * represents any weight between 1 and 2^32 (represented as zero).
 * When the weight is 2^32, the stable name table owns "all" of the
 * stable pointers to this object, and the entry can be garbage
 * collected if the object isn't reachable.
 *
 * A new stable pointer is given the weight log2(W/2), where W is the
 * weight stored in the table entry.  The new weight in the table is W
 * - 2^log2(W/2).
 *
 * A stable pointer can be "split" into two stable pointers, by
 * dividing the weight by 2 and giving each pointer half.
 * When freeing a stable pointer, the weight of the pointer is added
 * to the weight stored in the table entry.
 * */

118
static HashTable *addrToStableHash = NULL;
119 120 121

#define INIT_SPT_SIZE 64

sof's avatar
sof committed
122
STATIC_INLINE void
123 124 125 126 127
initFreeList(snEntry *table, nat n, snEntry *free)
{
  snEntry *p;

  for (p = table + n - 1; p >= table; p--) {
sof's avatar
sof committed
128
    p->addr   = (P_)free;
129
    p->old    = NULL;
130
    p->ref    = 0;
sof's avatar
sof committed
131
    p->sn_obj = NULL;
132 133 134 135 136 137 138 139
    free = p;
  }
  stable_ptr_free = table;
}

void
initStablePtrTable(void)
{
140 141 142 143 144 145 146 147 148 149 150 151 152 153
	if (SPT_size > 0)
		return;

    SPT_size = INIT_SPT_SIZE;
    stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
				      "initStablePtrTable");

    /* we don't use index 0 in the stable name table, because that
     * would conflict with the hash table lookup operations which
     * return NULL if an entry isn't found in the hash table.
     */
    initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
    addrToStableHash = allocHashTable();

154
#ifdef THREADED_RTS
155
    initMutex(&stable_mutex);
156
#endif
157 158
}

159 160 161 162 163 164 165 166 167 168
void
exitStablePtrTable(void)
{
  if (addrToStableHash)
    freeHashTable(addrToStableHash, NULL);
  addrToStableHash = NULL;
  if (stable_ptr_table)
    stgFree(stable_ptr_table);
  stable_ptr_table = NULL;
  SPT_size = 0;
169 170 171
#ifdef THREADED_RTS
  closeMutex(&stable_mutex);
#endif
172 173
}

sof's avatar
sof committed
174 175
/*
 * get at the real stuff...remove indirections.
Simon Marlow's avatar
Simon Marlow committed
176 177 178
 * It untags pointers before dereferencing and
 * retags the real stuff with its tag (if there
 * is any) when returning.
sof's avatar
sof committed
179 180 181 182 183 184 185
 *
 * ToDo: move to a better home.
 */
static
StgClosure*
removeIndirections(StgClosure* p)
{
Simon Marlow's avatar
Simon Marlow committed
186 187
  StgWord tag = GET_CLOSURE_TAG(p);
  StgClosure* q = UNTAG_CLOSURE(p);
sof's avatar
sof committed
188

189 190
  while (get_itbl(q)->type == IND ||
         get_itbl(q)->type == IND_STATIC ||
191
         get_itbl(q)->type == IND_PERM) {
192
      q = ((StgInd *)q)->indirectee;
Simon Marlow's avatar
Simon Marlow committed
193
      tag = GET_CLOSURE_TAG(q);
194
      q = UNTAG_CLOSURE(q);
sof's avatar
sof committed
195
  }
196

Simon Marlow's avatar
Simon Marlow committed
197
  return TAG_CLOSURE(tag,q);
sof's avatar
sof committed
198 199
}

200 201
static StgWord
lookupStableName_(StgPtr p)
202 203
{
  StgWord sn;
204
  void* sn_tmp;
205 206 207 208

  if (stable_ptr_free == NULL) {
    enlargeStablePtrTable();
  }
sof's avatar
sof committed
209 210

  /* removing indirections increases the likelihood
211
   * of finding a match in the stable name hash table.
sof's avatar
sof committed
212 213 214
   */
  p = (StgPtr)removeIndirections((StgClosure*)p);

Simon Marlow's avatar
Simon Marlow committed
215 216 217
  // register the untagged pointer.  This just makes things simpler.
  p = (StgPtr)UNTAG_CLOSURE((StgClosure*)p);

218 219
  sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
  sn = (StgWord)sn_tmp;
220 221 222
  
  if (sn != 0) {
    ASSERT(stable_ptr_table[sn].addr == p);
Simon Marlow's avatar
Simon Marlow committed
223
    debugTrace(DEBUG_stable, "cached stable name %ld at %p",sn,p);
224 225 226
    return sn;
  } else {
    sn = stable_ptr_free - stable_ptr_table;
227
    stable_ptr_free  = (snEntry*)(stable_ptr_free->addr);
228
    stable_ptr_table[sn].ref = 0;
229
    stable_ptr_table[sn].addr = p;
230
    stable_ptr_table[sn].sn_obj = NULL;
Simon Marlow's avatar
Simon Marlow committed
231
    /* debugTrace(DEBUG_stable, "new stable name %d at %p\n",sn,p); */
232 233 234 235 236 237 238 239
    
    /* add the new stable name to the hash table */
    insertHashTable(addrToStableHash, (W_)p, (void *)sn);

    return sn;
  }
}

240 241 242 243
StgWord
lookupStableName(StgPtr p)
{
    StgWord res;
244 245

    initStablePtrTable();
246 247 248 249 250 251
    ACQUIRE_LOCK(&stable_mutex);
    res = lookupStableName_(p);
    RELEASE_LOCK(&stable_mutex);
    return res;
}

sof's avatar
sof committed
252
STATIC_INLINE void
253 254
freeStableName(snEntry *sn)
{
255
  ASSERT(sn->sn_obj == NULL);
256
  if (sn->addr != NULL) {
257
      removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
258
  }
259 260 261 262 263 264 265
  sn->addr = (P_)stable_ptr_free;
  stable_ptr_free = sn;
}

StgStablePtr
getStablePtr(StgPtr p)
{
266 267
  StgWord sn;

268
  initStablePtrTable();
269 270
  ACQUIRE_LOCK(&stable_mutex);
  sn = lookupStableName_(p);
271
  stable_ptr_table[sn].ref++;
272
  RELEASE_LOCK(&stable_mutex);
273
  return (StgStablePtr)(sn);
274 275
}

276 277 278
void
freeStablePtr(StgStablePtr sp)
{
279 280
    snEntry *sn;

281
	initStablePtrTable();
282 283 284
    ACQUIRE_LOCK(&stable_mutex);

    sn = &stable_ptr_table[(StgWord)sp];
285
    
286 287 288 289 290 291 292
    ASSERT((StgWord)sp < SPT_size  &&  sn->addr != NULL  &&  sn->ref > 0);

    sn->ref--;

    // If this entry has no StableName attached, then just free it
    // immediately.  This is important; it might be a while before the
    // next major GC which actually collects the entry.
293
    if (sn->sn_obj == NULL && sn->ref == 0) {
294 295
	freeStableName(sn);
    }
296 297

    RELEASE_LOCK(&stable_mutex);
298 299
}

Simon Marlow's avatar
Simon Marlow committed
300
static void
301 302 303
enlargeStablePtrTable(void)
{
  nat old_SPT_size = SPT_size;
304

305
    // 2nd and subsequent times
306 307 308
  SPT_size *= 2;
  stable_ptr_table =
    stgReallocBytes(stable_ptr_table,
sof's avatar
sof committed
309
		      SPT_size * sizeof(snEntry),
310
		      "enlargeStablePtrTable");
sof's avatar
sof committed
311

312
  initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
313 314
}

315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331
/* -----------------------------------------------------------------------------
 * We must lock the StablePtr table during GC, to prevent simultaneous
 * calls to freeStablePtr().
 * -------------------------------------------------------------------------- */

void
stablePtrPreGC(void)
{
    ACQUIRE_LOCK(&stable_mutex);
}

void
stablePtrPostGC(void)
{
    RELEASE_LOCK(&stable_mutex);
}

332 333 334
/* -----------------------------------------------------------------------------
 * Treat stable pointers as roots for the garbage collector.
 *
335
 * A stable pointer is any stable name entry with a ref > 0.  We'll
336 337 338 339
 * take the opportunity to zero the "keep" flags at the same time.
 * -------------------------------------------------------------------------- */

void
340
markStablePtrTable(evac_fn evac, void *user)
341
{
342 343 344 345 346 347 348 349 350 351
    snEntry *p, *end_stable_ptr_table;
    StgPtr q;
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    // Mark all the stable *pointers* (not stable names).
    // _starting_ at index 1; index 0 is unused.
    for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
	q = p->addr;

352 353 354
	// Internal pointers are free slots.  If q == NULL, it's a
	// stable name where the object has been GC'd, but the
	// StableName object (sn_obj) is still alive.
355 356 357 358 359 360 361
	if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {

	    // save the current addr away: we need to be able to tell
	    // whether the objects moved in order to be able to update
	    // the hash table later.
	    p->old = p->addr;

362 363
	    // if the ref is non-zero, treat addr as a root
	    if (p->ref != 0) {
364
		evac(user, (StgClosure **)&p->addr);
365 366 367 368
	    }
	}
    }
}
369

370 371 372 373 374 375 376
/* -----------------------------------------------------------------------------
 * Thread the stable pointer table for compacting GC.
 * 
 * Here we must call the supplied evac function for each pointer into
 * the heap from the stable pointer table, because the compacting
 * collector may move the object it points to.
 * -------------------------------------------------------------------------- */
377

378
void
379
threadStablePtrTable( evac_fn evac, void *user )
380 381 382 383 384 385 386 387
{
    snEntry *p, *end_stable_ptr_table;
    StgPtr q;
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
	
388
	if (p->sn_obj != NULL) {
389
	    evac(user, (StgClosure **)&p->sn_obj);
390 391 392
	}

	q = p->addr;
393
	if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
394
	    evac(user, (StgClosure **)&p->addr);
395 396 397 398 399 400 401 402 403
	}
    }
}

/* -----------------------------------------------------------------------------
 * Garbage collect any dead entries in the stable pointer table.
 *
 * A dead entry has:
 *
404
 *          - a zero reference count
405
 *          - a dead sn_obj
406
 *
407 408 409 410
 * Both of these conditions must be true in order to re-use the stable
 * name table entry.  We can re-use stable name table entries for live
 * heap objects, as long as the program has no StableName objects that
 * refer to the entry.
411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428
 * -------------------------------------------------------------------------- */

void
gcStablePtrTable( void )
{
    snEntry *p, *end_stable_ptr_table;
    StgPtr q;
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    // NOTE: _starting_ at index 1; index 0 is unused.
    for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
	
	// Update the pointer to the StableName object, if there is one
	if (p->sn_obj != NULL) {
	    p->sn_obj = isAlive(p->sn_obj);
	}
	
429 430 431
	// Internal pointers are free slots.  If q == NULL, it's a
	// stable name where the object has been GC'd, but the
	// StableName object (sn_obj) is still alive.
432 433 434 435
	q = p->addr;
	if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {

	    // StableNames only:
436
	    if (p->ref == 0) {
437 438 439
		if (p->sn_obj == NULL) {
		    // StableName object is dead
		    freeStableName(p);
440 441
		    debugTrace(DEBUG_stable, "GC'd Stable name %ld",
			       (long)(p - stable_ptr_table));
442 443 444
		    continue;
		    
		} else {
445
		  p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
Simon Marlow's avatar
Simon Marlow committed
446 447
		  debugTrace(DEBUG_stable, 
			     "stable name %ld still alive at %p, ref %ld\n",
448
			     (long)(p - stable_ptr_table), p->addr, p->ref);
449 450 451 452 453 454 455 456
		}
	    }
	}
    }
}

/* -----------------------------------------------------------------------------
 * Update the StablePtr/StableName hash table
457 458 459 460 461 462 463 464
 *
 * The boolean argument 'full' indicates that a major collection is
 * being done, so we might as well throw away the hash table and build
 * a new one.  For a minor collection, we just re-hash the elements
 * that changed.
 * -------------------------------------------------------------------------- */

void
465
updateStablePtrTable(rtsBool full)
466
{
467 468 469 470 471
    snEntry *p, *end_stable_ptr_table;
    
    if (full && addrToStableHash != NULL) {
	freeHashTable(addrToStableHash,NULL);
	addrToStableHash = allocHashTable();
472
    }
473 474 475 476 477
    
    end_stable_ptr_table = &stable_ptr_table[SPT_size];
    
    // NOTE: _starting_ at index 1; index 0 is unused.
    for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
478
	
479 480 481 482 483 484 485 486 487 488 489
	if (p->addr == NULL) {
	    if (p->old != NULL) {
		// The target has been garbage collected.  Remove its
		// entry from the hash table.
		removeHashTable(addrToStableHash, (W_)p->old, NULL);
		p->old = NULL;
	    }
	}
	else if (p->addr < (P_)stable_ptr_table 
		 || p->addr >= (P_)end_stable_ptr_table) {
	    // Target still alive, Re-hash this stable name 
490
	    if (full) {
491 492 493 494 495 496
		insertHashTable(addrToStableHash, (W_)p->addr, 
				(void *)(p - stable_ptr_table));
	    } else if (p->addr != p->old) {
		removeHashTable(addrToStableHash, (W_)p->old, NULL);
		insertHashTable(addrToStableHash, (W_)p->addr, 
				(void *)(p - stable_ptr_table));
497
	    }
498 499 500
	}
    }
}