1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team 1998-2008
5 * Weak pointers and weak-like things in the GC
7 * Documentation on the architecture of the Garbage Collector can be
8 * found in the online commentary:
10 * http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC
12 * ---------------------------------------------------------------------------*/
23 /* -----------------------------------------------------------------------------
26 traverse_weak_ptr_list is called possibly many times during garbage
27 collection. It returns a flag indicating whether it did any work
28 (i.e. called evacuate on any live pointers).
30 Invariant: traverse_weak_ptr_list is called when the heap is in an
31 idempotent state. That means that there are no pending
32 evacuate/scavenge operations. This invariant helps the weak
33 pointer code decide which weak pointers are dead - if there are no
34 new live weak pointers, then all the currently unreachable ones are
37 For generational GC: we just don't try to finalize weak pointers in
38 older generations than the one we're collecting. This could
39 probably be optimised by keeping per-generation lists of weak
40 pointers, but for a few weak pointers this scheme will work.
42 There are three distinct stages to processing weak pointers:
44 - weak_stage == WeakPtrs
46 We process all the weak pointers whos keys are alive (evacuate
47 their values and finalizers), and repeat until we can find no new
48 live keys. If no live keys are found in this pass, then we
49 evacuate the finalizers of all the dead weak pointers in order to
52 - weak_stage == WeakThreads
54 Now, we discover which *threads* are still alive. Pointers to
55 threads from the all_threads and main thread lists are the
56 weakest of all: a pointers from the finalizer of a dead weak
57 pointer can keep a thread alive. Any threads found to be unreachable
58 are evacuated and placed on the resurrected_threads list so we
59 can send them a signal later.
61 - weak_stage == WeakDone
63 No more evacuation is done.
65 -------------------------------------------------------------------------- */
67 /* Which stage of processing various kinds of weak pointer are we at?
68 * (see traverse_weak_ptr_list() below for discussion).
70 typedef enum { WeakPtrs, WeakThreads, WeakDone } WeakStage;
71 static WeakStage weak_stage;
75 StgWeak *old_weak_ptr_list; // also pending finaliser list
77 /* List of all threads during GC
79 StgTSO *resurrected_threads;
80 static StgTSO *old_all_threads;
85 old_weak_ptr_list = weak_ptr_list;
87 weak_stage = WeakPtrs;
89 /* The all_threads list is like the weak_ptr_list.
90 * See traverseWeakPtrList() for the details.
92 old_all_threads = all_threads;
93 all_threads = END_TSO_QUEUE;
94 resurrected_threads = END_TSO_QUEUE;
98 traverseWeakPtrList(void)
100 StgWeak *w, **last_w, *next_w;
102 rtsBool flag = rtsFalse;
104 switch (weak_stage) {
110 /* doesn't matter where we evacuate values/finalizers to, since
111 * these pointers are treated as roots (iff the keys are alive).
115 last_w = &old_weak_ptr_list;
116 for (w = old_weak_ptr_list; w != NULL; w = next_w) {
118 /* There might be a DEAD_WEAK on the list if finalizeWeak# was
119 * called on a live weak pointer object. Just remove it.
121 if (w->header.info == &stg_DEAD_WEAK_info) {
122 next_w = ((StgDeadWeak *)w)->link;
127 switch (get_itbl(w)->type) {
130 next_w = (StgWeak *)((StgEvacuated *)w)->evacuee;
135 /* Now, check whether the key is reachable.
137 new = isAlive(w->key);
140 // evacuate the value and finalizer
142 evacuate(&w->finalizer);
143 // remove this weak ptr from the old_weak_ptr list
145 // and put it on the new weak ptr list
147 w->link = weak_ptr_list;
151 debugTrace(DEBUG_weak,
152 "weak pointer still alive at %p -> %p",
163 barf("traverseWeakPtrList: not WEAK");
167 /* If we didn't make any changes, then we can go round and kill all
168 * the dead weak pointers. The old_weak_ptr list is used as a list
169 * of pending finalizers later on.
171 if (flag == rtsFalse) {
172 for (w = old_weak_ptr_list; w; w = w->link) {
173 evacuate(&w->finalizer);
176 // Next, move to the WeakThreads stage after fully
177 // scavenging the finalizers we've just evacuated.
178 weak_stage = WeakThreads;
184 /* Now deal with the all_threads list, which behaves somewhat like
185 * the weak ptr list. If we discover any threads that are about to
186 * become garbage, we wake them up and administer an exception.
189 StgTSO *t, *tmp, *next, **prev;
191 prev = &old_all_threads;
192 for (t = old_all_threads; t != END_TSO_QUEUE; t = next) {
194 tmp = (StgTSO *)isAlive((StgClosure *)t);
200 ASSERT(get_itbl(t)->type == TSO);
201 switch (t->what_next) {
202 case ThreadRelocated:
208 // finshed or died. The thread might still be alive, but we
209 // don't keep it on the all_threads list. Don't forget to
210 // stub out its global_link field.
211 next = t->global_link;
212 t->global_link = END_TSO_QUEUE;
220 // not alive (yet): leave this thread on the
221 // old_all_threads list.
222 prev = &(t->global_link);
223 next = t->global_link;
226 // alive: move this thread onto the all_threads list.
227 next = t->global_link;
228 t->global_link = all_threads;
235 /* If we evacuated any threads, we need to go back to the scavenger.
237 if (flag) return rtsTrue;
239 /* And resurrect any threads which were about to become garbage.
242 StgTSO *t, *tmp, *next;
243 for (t = old_all_threads; t != END_TSO_QUEUE; t = next) {
244 next = t->global_link;
246 evacuate((StgClosure **)&tmp);
247 tmp->global_link = resurrected_threads;
248 resurrected_threads = tmp;
252 /* Finally, we can update the blackhole_queue. This queue
253 * simply strings together TSOs blocked on black holes, it is
254 * not intended to keep anything alive. Hence, we do not follow
255 * pointers on the blackhole_queue until now, when we have
256 * determined which TSOs are otherwise reachable. We know at
257 * this point that all TSOs have been evacuated, however.
261 for (pt = &blackhole_queue; *pt != END_TSO_QUEUE; pt = &((*pt)->link)) {
262 *pt = (StgTSO *)isAlive((StgClosure *)*pt);
267 weak_stage = WeakDone; // *now* we're done,
268 return rtsTrue; // but one more round of scavenging, please
271 barf("traverse_weak_ptr_list");
277 /* -----------------------------------------------------------------------------
280 Threads on this list behave like weak pointers during the normal
281 phase of garbage collection: if the blackhole is reachable, then
282 the thread is reachable too.
283 -------------------------------------------------------------------------- */
285 traverseBlackholeQueue (void)
287 StgTSO *prev, *t, *tmp;
294 for (t = blackhole_queue; t != END_TSO_QUEUE; prev=t, t = t->link) {
295 // if the thread is not yet alive...
296 if (! (tmp = (StgTSO *)isAlive((StgClosure*)t))) {
297 // if the closure it is blocked on is either (a) a
298 // reachable BLAKCHOLE or (b) not a BLACKHOLE, then we
299 // make the thread alive.
300 if (!isAlive(t->block_info.closure)) {
301 type = get_itbl(t->block_info.closure)->type;
302 if (type == BLACKHOLE || type == CAF_BLACKHOLE) {
307 evacuate((StgClosure **)&tmp);
308 if (prev) prev->link = t;
315 /* -----------------------------------------------------------------------------
316 After GC, the live weak pointer list may have forwarding pointers
317 on it, because a weak pointer object was evacuated after being
318 moved to the live weak pointer list. We remove those forwarding
321 Also, we don't consider weak pointer objects to be reachable, but
322 we must nevertheless consider them to be "live" and retain them.
323 Therefore any weak pointer objects which haven't as yet been
324 evacuated need to be evacuated now.
325 -------------------------------------------------------------------------- */
328 markWeakPtrList ( void )
330 StgWeak *w, **last_w, *tmp;
332 last_w = &weak_ptr_list;
333 for (w = weak_ptr_list; w; w = w->link) {
334 // w might be WEAK, EVACUATED, or DEAD_WEAK (actually CON_STATIC) here
335 ASSERT(w->header.info == &stg_DEAD_WEAK_info
336 || get_itbl(w)->type == WEAK || get_itbl(w)->type == EVACUATED);
338 evacuate((StgClosure **)&tmp);