Refactoring of the GC in preparation for parallel GC
[ghc.git] / rts / sm / MarkWeak.c
1 /* -----------------------------------------------------------------------------
2 *
3 * (c) The GHC Team 1998-2006
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 "Rts.h"
15 #include "Storage.h"
16 #include "MarkWeak.h"
17 #include "GC.h"
18 #include "Evac.h"
19 #include "Trace.h"
20 #include "Schedule.h"
21
22 /* -----------------------------------------------------------------------------
23 Weak Pointers
24
25 traverse_weak_ptr_list is called possibly many times during garbage
26 collection. It returns a flag indicating whether it did any work
27 (i.e. called evacuate on any live pointers).
28
29 Invariant: traverse_weak_ptr_list is called when the heap is in an
30 idempotent state. That means that there are no pending
31 evacuate/scavenge operations. This invariant helps the weak
32 pointer code decide which weak pointers are dead - if there are no
33 new live weak pointers, then all the currently unreachable ones are
34 dead.
35
36 For generational GC: we just don't try to finalize weak pointers in
37 older generations than the one we're collecting. This could
38 probably be optimised by keeping per-generation lists of weak
39 pointers, but for a few weak pointers this scheme will work.
40
41 There are three distinct stages to processing weak pointers:
42
43 - weak_stage == WeakPtrs
44
45 We process all the weak pointers whos keys are alive (evacuate
46 their values and finalizers), and repeat until we can find no new
47 live keys. If no live keys are found in this pass, then we
48 evacuate the finalizers of all the dead weak pointers in order to
49 run them.
50
51 - weak_stage == WeakThreads
52
53 Now, we discover which *threads* are still alive. Pointers to
54 threads from the all_threads and main thread lists are the
55 weakest of all: a pointers from the finalizer of a dead weak
56 pointer can keep a thread alive. Any threads found to be unreachable
57 are evacuated and placed on the resurrected_threads list so we
58 can send them a signal later.
59
60 - weak_stage == WeakDone
61
62 No more evacuation is done.
63
64 -------------------------------------------------------------------------- */
65
66 /* Which stage of processing various kinds of weak pointer are we at?
67 * (see traverse_weak_ptr_list() below for discussion).
68 */
69 typedef enum { WeakPtrs, WeakThreads, WeakDone } WeakStage;
70 static WeakStage weak_stage;
71
72 /* Weak pointers
73 */
74 StgWeak *old_weak_ptr_list; // also pending finaliser list
75
76 /* List of all threads during GC
77 */
78 StgTSO *resurrected_threads;
79 static StgTSO *old_all_threads;
80
81 void
82 initWeakForGC(void)
83 {
84 old_weak_ptr_list = weak_ptr_list;
85 weak_ptr_list = NULL;
86 weak_stage = WeakPtrs;
87
88 /* The all_threads list is like the weak_ptr_list.
89 * See traverseWeakPtrList() for the details.
90 */
91 old_all_threads = all_threads;
92 all_threads = END_TSO_QUEUE;
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
103 switch (weak_stage) {
104
105 case WeakDone:
106 return rtsFalse;
107
108 case WeakPtrs:
109 /* doesn't matter where we evacuate values/finalizers to, since
110 * these pointers are treated as roots (iff the keys are alive).
111 */
112 gct->evac_gen = 0;
113
114 last_w = &old_weak_ptr_list;
115 for (w = old_weak_ptr_list; w != NULL; w = next_w) {
116
117 /* There might be a DEAD_WEAK on the list if finalizeWeak# was
118 * called on a live weak pointer object. Just remove it.
119 */
120 if (w->header.info == &stg_DEAD_WEAK_info) {
121 next_w = ((StgDeadWeak *)w)->link;
122 *last_w = next_w;
123 continue;
124 }
125
126 switch (get_itbl(w)->type) {
127
128 case EVACUATED:
129 next_w = (StgWeak *)((StgEvacuated *)w)->evacuee;
130 *last_w = next_w;
131 continue;
132
133 case WEAK:
134 /* Now, check whether the key is reachable.
135 */
136 new = isAlive(w->key);
137 if (new != NULL) {
138 w->key = new;
139 // evacuate the value and finalizer
140 w->value = evacuate(w->value);
141 w->finalizer = evacuate(w->finalizer);
142 // remove this weak ptr from the old_weak_ptr list
143 *last_w = w->link;
144 // and put it on the new weak ptr list
145 next_w = w->link;
146 w->link = weak_ptr_list;
147 weak_ptr_list = w;
148 flag = rtsTrue;
149
150 debugTrace(DEBUG_weak,
151 "weak pointer still alive at %p -> %p",
152 w, w->key);
153 continue;
154 }
155 else {
156 last_w = &(w->link);
157 next_w = w->link;
158 continue;
159 }
160
161 default:
162 barf("traverseWeakPtrList: not WEAK");
163 }
164 }
165
166 /* If we didn't make any changes, then we can go round and kill all
167 * the dead weak pointers. The old_weak_ptr list is used as a list
168 * of pending finalizers later on.
169 */
170 if (flag == rtsFalse) {
171 for (w = old_weak_ptr_list; w; w = w->link) {
172 w->finalizer = evacuate(w->finalizer);
173 }
174
175 // Next, move to the WeakThreads stage after fully
176 // scavenging the finalizers we've just evacuated.
177 weak_stage = WeakThreads;
178 }
179
180 return rtsTrue;
181
182 case WeakThreads:
183 /* Now deal with the all_threads list, which behaves somewhat like
184 * the weak ptr list. If we discover any threads that are about to
185 * become garbage, we wake them up and administer an exception.
186 */
187 {
188 StgTSO *t, *tmp, *next, **prev;
189
190 prev = &old_all_threads;
191 for (t = old_all_threads; t != END_TSO_QUEUE; t = next) {
192
193 tmp = (StgTSO *)isAlive((StgClosure *)t);
194
195 if (tmp != NULL) {
196 t = tmp;
197 }
198
199 ASSERT(get_itbl(t)->type == TSO);
200 switch (t->what_next) {
201 case ThreadRelocated:
202 next = t->link;
203 *prev = next;
204 continue;
205 case ThreadKilled:
206 case ThreadComplete:
207 // finshed or died. The thread might still be alive, but we
208 // don't keep it on the all_threads list. Don't forget to
209 // stub out its global_link field.
210 next = t->global_link;
211 t->global_link = END_TSO_QUEUE;
212 *prev = next;
213 continue;
214 default:
215 ;
216 }
217
218 if (tmp == NULL) {
219 // not alive (yet): leave this thread on the
220 // old_all_threads list.
221 prev = &(t->global_link);
222 next = t->global_link;
223 }
224 else {
225 // alive: move this thread onto the all_threads list.
226 next = t->global_link;
227 t->global_link = all_threads;
228 all_threads = t;
229 *prev = next;
230 }
231 }
232 }
233
234 /* If we evacuated any threads, we need to go back to the scavenger.
235 */
236 if (flag) return rtsTrue;
237
238 /* And resurrect any threads which were about to become garbage.
239 */
240 {
241 StgTSO *t, *tmp, *next;
242 for (t = old_all_threads; t != END_TSO_QUEUE; t = next) {
243 next = t->global_link;
244 tmp = (StgTSO *)evacuate((StgClosure *)t);
245 tmp->global_link = resurrected_threads;
246 resurrected_threads = tmp;
247 }
248 }
249
250 /* Finally, we can update the blackhole_queue. This queue
251 * simply strings together TSOs blocked on black holes, it is
252 * not intended to keep anything alive. Hence, we do not follow
253 * pointers on the blackhole_queue until now, when we have
254 * determined which TSOs are otherwise reachable. We know at
255 * this point that all TSOs have been evacuated, however.
256 */
257 {
258 StgTSO **pt;
259 for (pt = &blackhole_queue; *pt != END_TSO_QUEUE; pt = &((*pt)->link)) {
260 *pt = (StgTSO *)isAlive((StgClosure *)*pt);
261 ASSERT(*pt != NULL);
262 }
263 }
264
265 weak_stage = WeakDone; // *now* we're done,
266 return rtsTrue; // but one more round of scavenging, please
267
268 default:
269 barf("traverse_weak_ptr_list");
270 return rtsTrue;
271 }
272
273 }
274
275 /* -----------------------------------------------------------------------------
276 The blackhole queue
277
278 Threads on this list behave like weak pointers during the normal
279 phase of garbage collection: if the blackhole is reachable, then
280 the thread is reachable too.
281 -------------------------------------------------------------------------- */
282 rtsBool
283 traverseBlackholeQueue (void)
284 {
285 StgTSO *prev, *t, *tmp;
286 rtsBool flag;
287 nat type;
288
289 flag = rtsFalse;
290 prev = NULL;
291
292 for (t = blackhole_queue; t != END_TSO_QUEUE; prev=t, t = t->link) {
293 // if the thread is not yet alive...
294 if (! (tmp = (StgTSO *)isAlive((StgClosure*)t))) {
295 // if the closure it is blocked on is either (a) a
296 // reachable BLAKCHOLE or (b) not a BLACKHOLE, then we
297 // make the thread alive.
298 if (!isAlive(t->block_info.closure)) {
299 type = get_itbl(t->block_info.closure)->type;
300 if (type == BLACKHOLE || type == CAF_BLACKHOLE) {
301 continue;
302 }
303 }
304 t = (StgTSO *)evacuate((StgClosure *)t);
305 if (prev) prev->link = t;
306 flag = rtsTrue;
307 }
308 }
309 return flag;
310 }
311
312 /* -----------------------------------------------------------------------------
313 After GC, the live weak pointer list may have forwarding pointers
314 on it, because a weak pointer object was evacuated after being
315 moved to the live weak pointer list. We remove those forwarding
316 pointers here.
317
318 Also, we don't consider weak pointer objects to be reachable, but
319 we must nevertheless consider them to be "live" and retain them.
320 Therefore any weak pointer objects which haven't as yet been
321 evacuated need to be evacuated now.
322 -------------------------------------------------------------------------- */
323
324 void
325 markWeakPtrList ( void )
326 {
327 StgWeak *w, **last_w;
328
329 last_w = &weak_ptr_list;
330 for (w = weak_ptr_list; w; w = w->link) {
331 // w might be WEAK, EVACUATED, or DEAD_WEAK (actually CON_STATIC) here
332 ASSERT(w->header.info == &stg_DEAD_WEAK_info
333 || get_itbl(w)->type == WEAK || get_itbl(w)->type == EVACUATED);
334 w = (StgWeak *)evacuate((StgClosure *)w);
335 *last_w = w;
336 last_w = &(w->link);
337 }
338 }
339