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;
99 const StgInfoTable *info;
101 switch (weak_stage) {
107 /* doesn't matter where we evacuate values/finalizers to, since
108 * these pointers are treated as roots (iff the keys are alive).
112 last_w = &old_weak_ptr_list;
113 for (w = old_weak_ptr_list; w != NULL; w = next_w) {
115 /* There might be a DEAD_WEAK on the list if finalizeWeak# was
116 * called on a live weak pointer object. Just remove it.
118 if (w->header.info == &stg_DEAD_WEAK_info) {
119 next_w = ((StgDeadWeak *)w)->link;
124 info = w->header.info;
125 if (IS_FORWARDING_PTR(info)) {
126 next_w = (StgWeak *)UN_FORWARDING_PTR(info);
131 switch (INFO_PTR_TO_STRUCT(info)->type) {
134 /* Now, check whether the key is reachable.
136 new = isAlive(w->key);
139 // evacuate the value and finalizer
141 evacuate(&w->finalizer);
142 // remove this weak ptr from the old_weak_ptr list
144 // and put it on the new weak ptr list
146 w->link = weak_ptr_list;
150 debugTrace(DEBUG_weak,
151 "weak pointer still alive at %p -> %p",
162 barf("traverseWeakPtrList: not WEAK");
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.
170 if (flag == rtsFalse) {
171 for (w = old_weak_ptr_list; w; w = w->link) {
172 evacuate(&w->finalizer);
175 // Next, move to the WeakThreads stage after fully
176 // scavenging the finalizers we've just evacuated.
177 weak_stage = 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.
188 StgTSO *t, *tmp, *next, **prev;
192 // Traverse thread lists for generations we collected...
193 for (g = 0; g <= N; g++) {
194 for (s = 0; s < generations[g].n_steps; s++) {
195 stp = &generations[g].steps[s];
197 prev = &stp->old_threads;
199 for (t = stp->old_threads; t != END_TSO_QUEUE; t = next) {
201 tmp = (StgTSO *)isAlive((StgClosure *)t);
207 ASSERT(get_itbl(t)->type == TSO);
208 if (t->what_next == ThreadRelocated) {
214 next = t->global_link;
216 // This is a good place to check for blocked
217 // exceptions. It might be the case that a thread is
218 // blocked on delivering an exception to a thread that
219 // is also blocked - we try to ensure that this
220 // doesn't happen in throwTo(), but it's too hard (or
221 // impossible) to close all the race holes, so we
222 // accept that some might get through and deal with
223 // them here. A GC will always happen at some point,
224 // even if the system is otherwise deadlocked.
226 // If an unreachable thread has blocked
227 // exceptions, we really want to perform the
228 // blocked exceptions rather than throwing
229 // BlockedIndefinitely exceptions. This is the
230 // only place we can discover such threads.
231 // The target thread might even be
232 // ThreadFinished or ThreadKilled. Bugs here
233 // will only be seen when running on a
235 if (t->blocked_exceptions != END_TSO_QUEUE) {
237 evacuate((StgClosure **)&t);
240 t->global_link = exception_threads;
241 exception_threads = t;
247 // not alive (yet): leave this thread on the
248 // old_all_threads list.
249 prev = &(t->global_link);
255 // move this thread onto the correct threads list.
257 new_step = Bdescr((P_)t)->step;
258 t->global_link = new_step->threads;
259 new_step->threads = t;
266 /* If we evacuated any threads, we need to go back to the scavenger.
268 if (flag) return rtsTrue;
270 /* And resurrect any threads which were about to become garbage.
275 StgTSO *t, *tmp, *next;
277 for (g = 0; g <= N; g++) {
278 for (s = 0; s < generations[g].n_steps; s++) {
279 stp = &generations[g].steps[s];
281 for (t = stp->old_threads; t != END_TSO_QUEUE; t = next) {
282 next = t->global_link;
284 // ThreadFinished and ThreadComplete: we have to keep
285 // these on the all_threads list until they
286 // become garbage, because they might get
287 // pending exceptions.
288 switch (t->what_next) {
294 evacuate((StgClosure **)&tmp);
295 tmp->global_link = resurrected_threads;
296 resurrected_threads = tmp;
303 /* Finally, we can update the blackhole_queue. This queue
304 * simply strings together TSOs blocked on black holes, it is
305 * not intended to keep anything alive. Hence, we do not follow
306 * pointers on the blackhole_queue until now, when we have
307 * determined which TSOs are otherwise reachable. We know at
308 * this point that all TSOs have been evacuated, however.
312 for (pt = &blackhole_queue; *pt != END_TSO_QUEUE; pt = &((*pt)->_link)) {
313 *pt = (StgTSO *)isAlive((StgClosure *)*pt);
318 weak_stage = WeakDone; // *now* we're done,
319 return rtsTrue; // but one more round of scavenging, please
322 barf("traverse_weak_ptr_list");
328 /* -----------------------------------------------------------------------------
331 Threads on this list behave like weak pointers during the normal
332 phase of garbage collection: if the blackhole is reachable, then
333 the thread is reachable too.
334 -------------------------------------------------------------------------- */
336 traverseBlackholeQueue (void)
338 StgTSO *prev, *t, *tmp;
345 for (t = blackhole_queue; t != END_TSO_QUEUE; prev=t, t = t->_link) {
346 // if the thread is not yet alive...
347 if (! (tmp = (StgTSO *)isAlive((StgClosure*)t))) {
348 // if the closure it is blocked on is either (a) a
349 // reachable BLAKCHOLE or (b) not a BLACKHOLE, then we
350 // make the thread alive.
351 if (!isAlive(t->block_info.closure)) {
352 type = get_itbl(t->block_info.closure)->type;
353 if (type == BLACKHOLE || type == CAF_BLACKHOLE) {
357 evacuate((StgClosure **)&t);
363 // no write barrier when on the blackhole queue,
364 // because we traverse the whole queue on every GC.
371 /* -----------------------------------------------------------------------------
372 After GC, the live weak pointer list may have forwarding pointers
373 on it, because a weak pointer object was evacuated after being
374 moved to the live weak pointer list. We remove those forwarding
377 Also, we don't consider weak pointer objects to be reachable, but
378 we must nevertheless consider them to be "live" and retain them.
379 Therefore any weak pointer objects which haven't as yet been
380 evacuated need to be evacuated now.
381 -------------------------------------------------------------------------- */
384 markWeakPtrList ( void )
386 StgWeak *w, **last_w, *tmp;
388 last_w = &weak_ptr_list;
389 for (w = weak_ptr_list; w; w = w->link) {
390 // w might be WEAK, EVACUATED, or DEAD_WEAK (actually CON_STATIC) here
391 ASSERT(IS_FORWARDING_PTR(w->header.info)
392 || w->header.info == &stg_DEAD_WEAK_info
393 || get_itbl(w)->type == WEAK);
395 evacuate((StgClosure **)&tmp);