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