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 threads found to be unreachable
78 StgTSO *resurrected_threads;
80 // List of blocked threads found to have pending throwTos
81 StgTSO *exception_threads;
86 old_weak_ptr_list = weak_ptr_list;
88 weak_stage = WeakPtrs;
89 resurrected_threads = END_TSO_QUEUE;
90 exception_threads = END_TSO_QUEUE;
94 traverseWeakPtrList(void)
96 StgWeak *w, **last_w, *next_w;
98 rtsBool flag = rtsFalse;
100 switch (weak_stage) {
106 /* doesn't matter where we evacuate values/finalizers to, since
107 * these pointers are treated as roots (iff the keys are alive).
111 last_w = &old_weak_ptr_list;
112 for (w = old_weak_ptr_list; w != NULL; w = next_w) {
114 /* There might be a DEAD_WEAK on the list if finalizeWeak# was
115 * called on a live weak pointer object. Just remove it.
117 if (w->header.info == &stg_DEAD_WEAK_info) {
118 next_w = ((StgDeadWeak *)w)->link;
123 switch (get_itbl(w)->type) {
126 next_w = (StgWeak *)((StgEvacuated *)w)->evacuee;
131 /* Now, check whether the key is reachable.
133 new = isAlive(w->key);
136 // evacuate the value and finalizer
138 evacuate(&w->finalizer);
139 // remove this weak ptr from the old_weak_ptr list
141 // and put it on the new weak ptr list
143 w->link = weak_ptr_list;
147 debugTrace(DEBUG_weak,
148 "weak pointer still alive at %p -> %p",
159 barf("traverseWeakPtrList: not WEAK");
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.
167 if (flag == rtsFalse) {
168 for (w = old_weak_ptr_list; w; w = w->link) {
169 evacuate(&w->finalizer);
172 // Next, move to the WeakThreads stage after fully
173 // scavenging the finalizers we've just evacuated.
174 weak_stage = WeakThreads;
180 /* Now deal with the all_threads list, which behaves 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.
185 StgTSO *t, *tmp, *next, **prev;
189 // Traverse thread lists for generations we collected...
190 for (g = 0; g <= N; g++) {
191 for (s = 0; s < generations[g].n_steps; s++) {
192 stp = &generations[g].steps[s];
194 prev = &stp->old_threads;
196 for (t = stp->old_threads; t != END_TSO_QUEUE; t = next) {
198 tmp = (StgTSO *)isAlive((StgClosure *)t);
204 ASSERT(get_itbl(t)->type == TSO);
205 switch (t->what_next) {
206 case ThreadRelocated:
212 // finshed or died. The thread might still
213 // be alive, but we don't keep it on the
214 // all_threads list. Don't forget to
215 // stub out its global_link field.
216 next = t->global_link;
217 t->global_link = END_TSO_QUEUE;
225 // not alive (yet): leave this thread on the
226 // old_all_threads list.
227 prev = &(t->global_link);
228 next = t->global_link;
232 next = t->global_link;
235 // This is a good place to check for blocked
236 // exceptions. It might be the case that a thread is
237 // blocked on delivering an exception to a thread that
238 // is also blocked - we try to ensure that this
239 // doesn't happen in throwTo(), but it's too hard (or
240 // impossible) to close all the race holes, so we
241 // accept that some might get through and deal with
242 // them here. A GC will always happen at some point,
243 // even if the system is otherwise deadlocked.
244 if (t->blocked_exceptions != END_TSO_QUEUE) {
245 t->global_link = exception_threads;
246 exception_threads = t;
248 // move this thread onto the correct threads list.
250 new_step = Bdescr((P_)t)->step;
251 t->global_link = new_step->threads;
252 new_step->threads = t;
260 /* If we evacuated any threads, we need to go back to the scavenger.
262 if (flag) return rtsTrue;
264 /* And resurrect any threads which were about to become garbage.
269 StgTSO *t, *tmp, *next;
271 for (g = 0; g <= N; g++) {
272 for (s = 0; s < generations[g].n_steps; s++) {
273 stp = &generations[g].steps[s];
275 for (t = stp->old_threads; t != END_TSO_QUEUE; t = next) {
276 next = t->global_link;
278 evacuate((StgClosure **)&tmp);
279 tmp->global_link = resurrected_threads;
280 resurrected_threads = tmp;
286 /* Finally, we can update the blackhole_queue. This queue
287 * simply strings together TSOs blocked on black holes, it is
288 * not intended to keep anything alive. Hence, we do not follow
289 * pointers on the blackhole_queue until now, when we have
290 * determined which TSOs are otherwise reachable. We know at
291 * this point that all TSOs have been evacuated, however.
295 for (pt = &blackhole_queue; *pt != END_TSO_QUEUE; pt = &((*pt)->_link)) {
296 *pt = (StgTSO *)isAlive((StgClosure *)*pt);
301 weak_stage = WeakDone; // *now* we're done,
302 return rtsTrue; // but one more round of scavenging, please
305 barf("traverse_weak_ptr_list");
311 /* -----------------------------------------------------------------------------
314 Threads on this list behave like weak pointers during the normal
315 phase of garbage collection: if the blackhole is reachable, then
316 the thread is reachable too.
317 -------------------------------------------------------------------------- */
319 traverseBlackholeQueue (void)
321 StgTSO *prev, *t, *tmp;
328 for (t = blackhole_queue; t != END_TSO_QUEUE; prev=t, t = t->_link) {
329 // if the thread is not yet alive...
330 if (! (tmp = (StgTSO *)isAlive((StgClosure*)t))) {
331 // if the closure it is blocked on is either (a) a
332 // reachable BLAKCHOLE or (b) not a BLACKHOLE, then we
333 // make the thread alive.
334 if (!isAlive(t->block_info.closure)) {
335 type = get_itbl(t->block_info.closure)->type;
336 if (type == BLACKHOLE || type == CAF_BLACKHOLE) {
340 evacuate((StgClosure **)&t);
341 if (prev) prev->_link = t;
342 // no write barrier when on the blackhole queue,
343 // because we traverse the whole queue on every GC.
350 /* -----------------------------------------------------------------------------
351 After GC, the live weak pointer list may have forwarding pointers
352 on it, because a weak pointer object was evacuated after being
353 moved to the live weak pointer list. We remove those forwarding
356 Also, we don't consider weak pointer objects to be reachable, but
357 we must nevertheless consider them to be "live" and retain them.
358 Therefore any weak pointer objects which haven't as yet been
359 evacuated need to be evacuated now.
360 -------------------------------------------------------------------------- */
363 markWeakPtrList ( void )
365 StgWeak *w, **last_w, *tmp;
367 last_w = &weak_ptr_list;
368 for (w = weak_ptr_list; w; w = w->link) {
369 // w might be WEAK, EVACUATED, or DEAD_WEAK (actually CON_STATIC) here
370 ASSERT(w->header.info == &stg_DEAD_WEAK_info
371 || get_itbl(w)->type == WEAK || get_itbl(w)->type == EVACUATED);
373 evacuate((StgClosure **)&tmp);