New implementation of BLACKHOLEs
[ghc.git] / rts / sm / MarkWeak.c
1 /* -----------------------------------------------------------------------------
2 *
3 * (c) The GHC Team 1998-2008
4 *
5 * Weak pointers and weak-like things in the GC
6 *
7 * Documentation on the architecture of the Garbage Collector can be
8 * found in the online commentary:
9 *
10 * http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC
11 *
12 * ---------------------------------------------------------------------------*/
13
14 #include "PosixSource.h"
15 #include "Rts.h"
16
17 #include "MarkWeak.h"
18 #include "GC.h"
19 #include "GCThread.h"
20 #include "Evac.h"
21 #include "Trace.h"
22 #include "Schedule.h"
23 #include "Weak.h"
24 #include "Storage.h"
25 #include "Threads.h"
26
27 /* -----------------------------------------------------------------------------
28 Weak Pointers
29
30 traverse_weak_ptr_list is called possibly many times during garbage
31 collection. It returns a flag indicating whether it did any work
32 (i.e. called evacuate on any live pointers).
33
34 Invariant: traverse_weak_ptr_list is called when the heap is in an
35 idempotent state. That means that there are no pending
36 evacuate/scavenge operations. This invariant helps the weak
37 pointer code decide which weak pointers are dead - if there are no
38 new live weak pointers, then all the currently unreachable ones are
39 dead.
40
41 For generational GC: we just don't try to finalize weak pointers in
42 older generations than the one we're collecting. This could
43 probably be optimised by keeping per-generation lists of weak
44 pointers, but for a few weak pointers this scheme will work.
45
46 There are three distinct stages to processing weak pointers:
47
48 - weak_stage == WeakPtrs
49
50 We process all the weak pointers whos keys are alive (evacuate
51 their values and finalizers), and repeat until we can find no new
52 live keys. If no live keys are found in this pass, then we
53 evacuate the finalizers of all the dead weak pointers in order to
54 run them.
55
56 - weak_stage == WeakThreads
57
58 Now, we discover which *threads* are still alive. Pointers to
59 threads from the all_threads and main thread lists are the
60 weakest of all: a pointers from the finalizer of a dead weak
61 pointer can keep a thread alive. Any threads found to be unreachable
62 are evacuated and placed on the resurrected_threads list so we
63 can send them a signal later.
64
65 - weak_stage == WeakDone
66
67 No more evacuation is done.
68
69 -------------------------------------------------------------------------- */
70
71 /* Which stage of processing various kinds of weak pointer are we at?
72 * (see traverse_weak_ptr_list() below for discussion).
73 */
74 typedef enum { WeakPtrs, WeakThreads, WeakDone } WeakStage;
75 static WeakStage weak_stage;
76
77 /* Weak pointers
78 */
79 StgWeak *old_weak_ptr_list; // also pending finaliser list
80
81 // List of threads found to be unreachable
82 StgTSO *resurrected_threads;
83
84 static void resurrectUnreachableThreads (generation *gen);
85 static rtsBool tidyThreadList (generation *gen);
86
87 void
88 initWeakForGC(void)
89 {
90 old_weak_ptr_list = weak_ptr_list;
91 weak_ptr_list = NULL;
92 weak_stage = WeakPtrs;
93 resurrected_threads = END_TSO_QUEUE;
94 }
95
96 rtsBool
97 traverseWeakPtrList(void)
98 {
99 StgWeak *w, **last_w, *next_w;
100 StgClosure *new;
101 rtsBool flag = rtsFalse;
102 const StgInfoTable *info;
103
104 switch (weak_stage) {
105
106 case WeakDone:
107 return rtsFalse;
108
109 case WeakPtrs:
110 /* doesn't matter where we evacuate values/finalizers to, since
111 * these pointers are treated as roots (iff the keys are alive).
112 */
113 gct->evac_gen = 0;
114
115 last_w = &old_weak_ptr_list;
116 for (w = old_weak_ptr_list; w != NULL; w = next_w) {
117
118 /* There might be a DEAD_WEAK on the list if finalizeWeak# was
119 * called on a live weak pointer object. Just remove it.
120 */
121 if (w->header.info == &stg_DEAD_WEAK_info) {
122 next_w = ((StgDeadWeak *)w)->link;
123 *last_w = next_w;
124 continue;
125 }
126
127 info = get_itbl(w);
128 switch (info->type) {
129
130 case WEAK:
131 /* Now, check whether the key is reachable.
132 */
133 new = isAlive(w->key);
134 if (new != NULL) {
135 w->key = new;
136 // evacuate the value and finalizer
137 evacuate(&w->value);
138 evacuate(&w->finalizer);
139 // remove this weak ptr from the old_weak_ptr list
140 *last_w = w->link;
141 // and put it on the new weak ptr list
142 next_w = w->link;
143 w->link = weak_ptr_list;
144 weak_ptr_list = w;
145 flag = rtsTrue;
146
147 debugTrace(DEBUG_weak,
148 "weak pointer still alive at %p -> %p",
149 w, w->key);
150 continue;
151 }
152 else {
153 last_w = &(w->link);
154 next_w = w->link;
155 continue;
156 }
157
158 default:
159 barf("traverseWeakPtrList: not WEAK");
160 }
161 }
162
163 /* If we didn't make any changes, then we can go round and kill all
164 * the dead weak pointers. The old_weak_ptr list is used as a list
165 * of pending finalizers later on.
166 */
167 if (flag == rtsFalse) {
168 for (w = old_weak_ptr_list; w; w = w->link) {
169 evacuate(&w->finalizer);
170 }
171
172 // Next, move to the WeakThreads stage after fully
173 // scavenging the finalizers we've just evacuated.
174 weak_stage = WeakThreads;
175 }
176
177 return rtsTrue;
178
179 case WeakThreads:
180 /* Now deal with the step->threads lists, which behave somewhat like
181 * the weak ptr list. If we discover any threads that are about to
182 * become garbage, we wake them up and administer an exception.
183 */
184 {
185 nat g;
186
187 // Traverse thread lists for generations we collected...
188 // ToDo when we have one gen per capability:
189 // for (n = 0; n < n_capabilities; n++) {
190 // if (tidyThreadList(&nurseries[n])) {
191 // flag = rtsTrue;
192 // }
193 // }
194 for (g = 0; g <= N; g++) {
195 if (tidyThreadList(&generations[g])) {
196 flag = rtsTrue;
197 }
198 }
199
200 /* If we evacuated any threads, we need to go back to the scavenger.
201 */
202 if (flag) return rtsTrue;
203
204 /* And resurrect any threads which were about to become garbage.
205 */
206 {
207 nat g;
208 for (g = 0; g <= N; g++) {
209 resurrectUnreachableThreads(&generations[g]);
210 }
211 }
212
213 weak_stage = WeakDone; // *now* we're done,
214 return rtsTrue; // but one more round of scavenging, please
215 }
216
217 default:
218 barf("traverse_weak_ptr_list");
219 return rtsTrue;
220 }
221 }
222
223 static void resurrectUnreachableThreads (generation *gen)
224 {
225 StgTSO *t, *tmp, *next;
226
227 for (t = gen->old_threads; t != END_TSO_QUEUE; t = next) {
228 next = t->global_link;
229
230 // ThreadFinished and ThreadComplete: we have to keep
231 // these on the all_threads list until they
232 // become garbage, because they might get
233 // pending exceptions.
234 switch (t->what_next) {
235 case ThreadKilled:
236 case ThreadComplete:
237 continue;
238 default:
239 tmp = t;
240 evacuate((StgClosure **)&tmp);
241 tmp->global_link = resurrected_threads;
242 resurrected_threads = tmp;
243 }
244 }
245 }
246
247 static rtsBool tidyThreadList (generation *gen)
248 {
249 StgTSO *t, *tmp, *next, **prev;
250 rtsBool flag = rtsFalse;
251
252 prev = &gen->old_threads;
253
254 for (t = gen->old_threads; t != END_TSO_QUEUE; t = next) {
255
256 tmp = (StgTSO *)isAlive((StgClosure *)t);
257
258 if (tmp != NULL) {
259 t = tmp;
260 }
261
262 ASSERT(get_itbl(t)->type == TSO);
263 if (t->what_next == ThreadRelocated) {
264 next = t->_link;
265 *prev = next;
266 continue;
267 }
268
269 next = t->global_link;
270
271 // if the thread is not masking exceptions but there are
272 // pending exceptions on its queue, then something has gone
273 // wrong:
274 ASSERT(t->blocked_exceptions == END_BLOCKED_EXCEPTIONS_QUEUE
275 || (t->flags & TSO_BLOCKEX));
276
277 if (tmp == NULL) {
278 // not alive (yet): leave this thread on the
279 // old_all_threads list.
280 prev = &(t->global_link);
281 }
282 else {
283 // alive
284 *prev = next;
285
286 // move this thread onto the correct threads list.
287 generation *new_gen;
288 new_gen = Bdescr((P_)t)->gen;
289 t->global_link = new_gen->threads;
290 new_gen->threads = t;
291 }
292 }
293
294 return flag;
295 }
296
297 /* -----------------------------------------------------------------------------
298 Evacuate every weak pointer object on the weak_ptr_list, and update
299 the link fields.
300
301 ToDo: with a lot of weak pointers, this will be expensive. We
302 should have a per-GC weak pointer list, just like threads.
303 -------------------------------------------------------------------------- */
304
305 void
306 markWeakPtrList ( void )
307 {
308 StgWeak *w, **last_w;
309
310 last_w = &weak_ptr_list;
311 for (w = weak_ptr_list; w; w = w->link) {
312 // w might be WEAK, EVACUATED, or DEAD_WEAK (actually CON_STATIC) here
313
314 #ifdef DEBUG
315 { // careful to do this assertion only reading the info ptr
316 // once, because during parallel GC it might change under our feet.
317 const StgInfoTable *info;
318 info = w->header.info;
319 ASSERT(IS_FORWARDING_PTR(info)
320 || info == &stg_DEAD_WEAK_info
321 || INFO_PTR_TO_STRUCT(info)->type == WEAK);
322 }
323 #endif
324
325 evacuate((StgClosure **)last_w);
326 w = *last_w;
327 if (w->header.info == &stg_DEAD_WEAK_info) {
328 last_w = &(((StgDeadWeak*)w)->link);
329 } else {
330 last_w = &(w->link);
331 }
332 }
333 }
334