Add closeMutex and use it on clean up
[ghc.git] / rts / Stable.c
1 /* -----------------------------------------------------------------------------
2 *
3 * (c) The GHC Team, 1998-2002
4 *
5 * Stable names and stable pointers.
6 *
7 * ---------------------------------------------------------------------------*/
8
9 // Make static versions of inline functions in Stable.h:
10 #define RTS_STABLE_C
11
12 #include "PosixSource.h"
13 #include "Rts.h"
14 #include "Hash.h"
15 #include "RtsUtils.h"
16 #include "OSThreads.h"
17 #include "Storage.h"
18 #include "RtsAPI.h"
19 #include "RtsFlags.h"
20 #include "OSThreads.h"
21 #include "Trace.h"
22
23 /* Comment from ADR's implementation in old RTS:
24
25 This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
26 small change in @HpOverflow.lc@) consists of the changes in the
27 runtime system required to implement "Stable Pointers". But we're
28 getting a bit ahead of ourselves --- what is a stable pointer and what
29 is it used for?
30
31 When Haskell calls C, it normally just passes over primitive integers,
32 floats, bools, strings, etc. This doesn't cause any problems at all
33 for garbage collection because the act of passing them makes a copy
34 from the heap, stack or wherever they are onto the C-world stack.
35 However, if we were to pass a heap object such as a (Haskell) @String@
36 and a garbage collection occured before we finished using it, we'd run
37 into problems since the heap object might have been moved or even
38 deleted.
39
40 So, if a C call is able to cause a garbage collection or we want to
41 store a pointer to a heap object between C calls, we must be careful
42 when passing heap objects. Our solution is to keep a table of all
43 objects we've given to the C-world and to make sure that the garbage
44 collector collects these objects --- updating the table as required to
45 make sure we can still find the object.
46
47
48 Of course, all this rather begs the question: why would we want to
49 pass a boxed value?
50
51 One very good reason is to preserve laziness across the language
52 interface. Rather than evaluating an integer or a string because it
53 {\em might\/} be required by the C function, we can wait until the C
54 function actually wants the value and then force an evaluation.
55
56 Another very good reason (the motivating reason!) is that the C code
57 might want to execute an object of sort $IO ()$ for the side-effects
58 it will produce. For example, this is used when interfacing to an X
59 widgets library to allow a direct implementation of callbacks.
60
61
62 The @makeStablePointer :: a -> IO (StablePtr a)@ function
63 converts a value into a stable pointer. It is part of the @PrimIO@
64 monad, because we want to be sure we don't allocate one twice by
65 accident, and then only free one of the copies.
66
67 \begin{verbatim}
68 makeStablePtr# :: a -> State# RealWorld -> (# RealWorld, a #)
69 freeStablePtr# :: StablePtr# a -> State# RealWorld -> State# RealWorld
70 deRefStablePtr# :: StablePtr# a -> State# RealWorld ->
71 (# State# RealWorld, a #)
72 \end{verbatim}
73
74 There may be additional functions on the C side to allow evaluation,
75 application, etc of a stable pointer.
76
77 */
78
79 snEntry *stable_ptr_table = NULL;
80 static snEntry *stable_ptr_free = NULL;
81
82 static unsigned int SPT_size = 0;
83
84 #ifdef THREADED_RTS
85 static Mutex stable_mutex;
86 #endif
87
88 /* This hash table maps Haskell objects to stable names, so that every
89 * call to lookupStableName on a given object will return the same
90 * stable name.
91 *
92 * OLD COMMENTS about reference counting follow. The reference count
93 * in a stable name entry is now just a counter.
94 *
95 * Reference counting
96 * ------------------
97 * A plain stable name entry has a zero reference count, which means
98 * the entry will dissappear when the object it points to is
99 * unreachable. For stable pointers, we need an entry that sticks
100 * around and keeps the object it points to alive, so each stable name
101 * entry has an associated reference count.
102 *
103 * A stable pointer has a weighted reference count N attached to it
104 * (actually in its upper 5 bits), which represents the weight
105 * 2^(N-1). The stable name entry keeps a 32-bit reference count, which
106 * represents any weight between 1 and 2^32 (represented as zero).
107 * When the weight is 2^32, the stable name table owns "all" of the
108 * stable pointers to this object, and the entry can be garbage
109 * collected if the object isn't reachable.
110 *
111 * A new stable pointer is given the weight log2(W/2), where W is the
112 * weight stored in the table entry. The new weight in the table is W
113 * - 2^log2(W/2).
114 *
115 * A stable pointer can be "split" into two stable pointers, by
116 * dividing the weight by 2 and giving each pointer half.
117 * When freeing a stable pointer, the weight of the pointer is added
118 * to the weight stored in the table entry.
119 * */
120
121 static HashTable *addrToStableHash = NULL;
122
123 #define INIT_SPT_SIZE 64
124
125 STATIC_INLINE void
126 initFreeList(snEntry *table, nat n, snEntry *free)
127 {
128 snEntry *p;
129
130 for (p = table + n - 1; p >= table; p--) {
131 p->addr = (P_)free;
132 p->old = NULL;
133 p->ref = 0;
134 p->sn_obj = NULL;
135 free = p;
136 }
137 stable_ptr_free = table;
138 }
139
140 void
141 initStablePtrTable(void)
142 {
143 if (SPT_size > 0)
144 return;
145
146 SPT_size = INIT_SPT_SIZE;
147 stable_ptr_table = stgMallocBytes(SPT_size * sizeof(snEntry),
148 "initStablePtrTable");
149
150 /* we don't use index 0 in the stable name table, because that
151 * would conflict with the hash table lookup operations which
152 * return NULL if an entry isn't found in the hash table.
153 */
154 initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
155 addrToStableHash = allocHashTable();
156
157 #ifdef THREADED_RTS
158 initMutex(&stable_mutex);
159 #endif
160 }
161
162 void
163 exitStablePtrTable(void)
164 {
165 if (addrToStableHash)
166 freeHashTable(addrToStableHash, NULL);
167 addrToStableHash = NULL;
168 if (stable_ptr_table)
169 stgFree(stable_ptr_table);
170 stable_ptr_table = NULL;
171 SPT_size = 0;
172 #ifdef THREADED_RTS
173 closeMutex(&stable_mutex);
174 #endif
175 }
176
177 /*
178 * get at the real stuff...remove indirections.
179 *
180 * ToDo: move to a better home.
181 */
182 static
183 StgClosure*
184 removeIndirections(StgClosure* p)
185 {
186 StgClosure* q = p;
187
188 while (get_itbl(q)->type == IND ||
189 get_itbl(q)->type == IND_STATIC ||
190 get_itbl(q)->type == IND_OLDGEN ||
191 get_itbl(q)->type == IND_PERM ||
192 get_itbl(q)->type == IND_OLDGEN_PERM ) {
193 q = ((StgInd *)q)->indirectee;
194 }
195 return q;
196 }
197
198 static StgWord
199 lookupStableName_(StgPtr p)
200 {
201 StgWord sn;
202 void* sn_tmp;
203
204 if (stable_ptr_free == NULL) {
205 enlargeStablePtrTable();
206 }
207
208 /* removing indirections increases the likelihood
209 * of finding a match in the stable name hash table.
210 */
211 p = (StgPtr)removeIndirections((StgClosure*)p);
212
213 sn_tmp = lookupHashTable(addrToStableHash,(W_)p);
214 sn = (StgWord)sn_tmp;
215
216 if (sn != 0) {
217 ASSERT(stable_ptr_table[sn].addr == p);
218 debugTrace(DEBUG_stable, "cached stable name %ld at %p",sn,p);
219 return sn;
220 } else {
221 sn = stable_ptr_free - stable_ptr_table;
222 stable_ptr_free = (snEntry*)(stable_ptr_free->addr);
223 stable_ptr_table[sn].ref = 0;
224 stable_ptr_table[sn].addr = p;
225 stable_ptr_table[sn].sn_obj = NULL;
226 /* debugTrace(DEBUG_stable, "new stable name %d at %p\n",sn,p); */
227
228 /* add the new stable name to the hash table */
229 insertHashTable(addrToStableHash, (W_)p, (void *)sn);
230
231 return sn;
232 }
233 }
234
235 StgWord
236 lookupStableName(StgPtr p)
237 {
238 StgWord res;
239
240 initStablePtrTable();
241 ACQUIRE_LOCK(&stable_mutex);
242 res = lookupStableName_(p);
243 RELEASE_LOCK(&stable_mutex);
244 return res;
245 }
246
247 STATIC_INLINE void
248 freeStableName(snEntry *sn)
249 {
250 ASSERT(sn->sn_obj == NULL);
251 if (sn->addr != NULL) {
252 removeHashTable(addrToStableHash, (W_)sn->addr, NULL);
253 }
254 sn->addr = (P_)stable_ptr_free;
255 stable_ptr_free = sn;
256 }
257
258 StgStablePtr
259 getStablePtr(StgPtr p)
260 {
261 StgWord sn;
262
263 initStablePtrTable();
264 ACQUIRE_LOCK(&stable_mutex);
265 sn = lookupStableName_(p);
266 stable_ptr_table[sn].ref++;
267 RELEASE_LOCK(&stable_mutex);
268 return (StgStablePtr)(sn);
269 }
270
271 void
272 freeStablePtr(StgStablePtr sp)
273 {
274 snEntry *sn;
275
276 initStablePtrTable();
277 ACQUIRE_LOCK(&stable_mutex);
278
279 sn = &stable_ptr_table[(StgWord)sp];
280
281 ASSERT((StgWord)sp < SPT_size && sn->addr != NULL && sn->ref > 0);
282
283 sn->ref--;
284
285 // If this entry has no StableName attached, then just free it
286 // immediately. This is important; it might be a while before the
287 // next major GC which actually collects the entry.
288 if (sn->sn_obj == NULL && sn->ref == 0) {
289 freeStableName(sn);
290 }
291
292 RELEASE_LOCK(&stable_mutex);
293 }
294
295 void
296 enlargeStablePtrTable(void)
297 {
298 nat old_SPT_size = SPT_size;
299
300 // 2nd and subsequent times
301 SPT_size *= 2;
302 stable_ptr_table =
303 stgReallocBytes(stable_ptr_table,
304 SPT_size * sizeof(snEntry),
305 "enlargeStablePtrTable");
306
307 initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
308 }
309
310 /* -----------------------------------------------------------------------------
311 * Treat stable pointers as roots for the garbage collector.
312 *
313 * A stable pointer is any stable name entry with a ref > 0. We'll
314 * take the opportunity to zero the "keep" flags at the same time.
315 * -------------------------------------------------------------------------- */
316
317 void
318 markStablePtrTable(evac_fn evac)
319 {
320 snEntry *p, *end_stable_ptr_table;
321 StgPtr q;
322
323 end_stable_ptr_table = &stable_ptr_table[SPT_size];
324
325 // Mark all the stable *pointers* (not stable names).
326 // _starting_ at index 1; index 0 is unused.
327 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
328 q = p->addr;
329
330 // Internal pointers are free slots. If q == NULL, it's a
331 // stable name where the object has been GC'd, but the
332 // StableName object (sn_obj) is still alive.
333 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
334
335 // save the current addr away: we need to be able to tell
336 // whether the objects moved in order to be able to update
337 // the hash table later.
338 p->old = p->addr;
339
340 // if the ref is non-zero, treat addr as a root
341 if (p->ref != 0) {
342 evac((StgClosure **)&p->addr);
343 }
344 }
345 }
346 }
347
348 /* -----------------------------------------------------------------------------
349 * Thread the stable pointer table for compacting GC.
350 *
351 * Here we must call the supplied evac function for each pointer into
352 * the heap from the stable pointer table, because the compacting
353 * collector may move the object it points to.
354 * -------------------------------------------------------------------------- */
355
356 void
357 threadStablePtrTable( evac_fn evac )
358 {
359 snEntry *p, *end_stable_ptr_table;
360 StgPtr q;
361
362 end_stable_ptr_table = &stable_ptr_table[SPT_size];
363
364 for (p = stable_ptr_table+1; p < end_stable_ptr_table; p++) {
365
366 if (p->sn_obj != NULL) {
367 evac((StgClosure **)&p->sn_obj);
368 }
369
370 q = p->addr;
371 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
372 evac((StgClosure **)&p->addr);
373 }
374 }
375 }
376
377 /* -----------------------------------------------------------------------------
378 * Garbage collect any dead entries in the stable pointer table.
379 *
380 * A dead entry has:
381 *
382 * - a zero reference count
383 * - a dead sn_obj
384 *
385 * Both of these conditions must be true in order to re-use the stable
386 * name table entry. We can re-use stable name table entries for live
387 * heap objects, as long as the program has no StableName objects that
388 * refer to the entry.
389 * -------------------------------------------------------------------------- */
390
391 void
392 gcStablePtrTable( void )
393 {
394 snEntry *p, *end_stable_ptr_table;
395 StgPtr q;
396
397 end_stable_ptr_table = &stable_ptr_table[SPT_size];
398
399 // NOTE: _starting_ at index 1; index 0 is unused.
400 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
401
402 // Update the pointer to the StableName object, if there is one
403 if (p->sn_obj != NULL) {
404 p->sn_obj = isAlive(p->sn_obj);
405 }
406
407 // Internal pointers are free slots. If q == NULL, it's a
408 // stable name where the object has been GC'd, but the
409 // StableName object (sn_obj) is still alive.
410 q = p->addr;
411 if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
412
413 // StableNames only:
414 if (p->ref == 0) {
415 if (p->sn_obj == NULL) {
416 // StableName object is dead
417 freeStableName(p);
418 debugTrace(DEBUG_stable, "GC'd Stable name %ld",
419 (long)(p - stable_ptr_table));
420 continue;
421
422 } else {
423 p->addr = (StgPtr)isAlive((StgClosure *)p->addr);
424 debugTrace(DEBUG_stable,
425 "stable name %ld still alive at %p, ref %ld\n",
426 (long)(p - stable_ptr_table), p->addr, p->ref);
427 }
428 }
429 }
430 }
431 }
432
433 /* -----------------------------------------------------------------------------
434 * Update the StablePtr/StableName hash table
435 *
436 * The boolean argument 'full' indicates that a major collection is
437 * being done, so we might as well throw away the hash table and build
438 * a new one. For a minor collection, we just re-hash the elements
439 * that changed.
440 * -------------------------------------------------------------------------- */
441
442 void
443 updateStablePtrTable(rtsBool full)
444 {
445 snEntry *p, *end_stable_ptr_table;
446
447 if (full && addrToStableHash != NULL) {
448 freeHashTable(addrToStableHash,NULL);
449 addrToStableHash = allocHashTable();
450 }
451
452 end_stable_ptr_table = &stable_ptr_table[SPT_size];
453
454 // NOTE: _starting_ at index 1; index 0 is unused.
455 for (p = stable_ptr_table + 1; p < end_stable_ptr_table; p++) {
456
457 if (p->addr == NULL) {
458 if (p->old != NULL) {
459 // The target has been garbage collected. Remove its
460 // entry from the hash table.
461 removeHashTable(addrToStableHash, (W_)p->old, NULL);
462 p->old = NULL;
463 }
464 }
465 else if (p->addr < (P_)stable_ptr_table
466 || p->addr >= (P_)end_stable_ptr_table) {
467 // Target still alive, Re-hash this stable name
468 if (full) {
469 insertHashTable(addrToStableHash, (W_)p->addr,
470 (void *)(p - stable_ptr_table));
471 } else if (p->addr != p->old) {
472 removeHashTable(addrToStableHash, (W_)p->old, NULL);
473 insertHashTable(addrToStableHash, (W_)p->addr,
474 (void *)(p - stable_ptr_table));
475 }
476 }
477 }
478 }