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clear any weak/soft/phantom references to finalizable objects before queuing

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If we don't clear these references, we risk finalizing objects which
can still be reached by one of the special reference types.

It's a bit of a chicken-and-egg problem.  We need to visit finalizable
objects before visiting weak references, since some of the weak
references and/or their targets may become reachable once the
finalizable objects are visited.  However, that ordering means we have
no efficient way of distinguishing between objects which are reachable
from one or more normal GC roots and those which are only reachable
via the finalization queue.  The solution is to clear all weak
references to finalizable objects before visiting them.
This commit is contained in:
Joel Dice 2012-10-05 10:06:01 -06:00
parent c680dd4215
commit f8d3494b1c
2 changed files with 44 additions and 2 deletions
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1
src/heap.cpp
View File

@ -1077,6 +1077,7 @@ update3(Context* c, void* o, bool* needsVisit)
fprintf(stderr, "mark fixie %p\n", f);
}
f->marked(true);
f->dead(false);
f->move(c, &(c->markedFixies));
}
*needsVisit = false;

45
src/machine.cpp
View File

@ -474,6 +474,25 @@ referenceTargetReachable(Thread* t, Heap::Visitor* v, object* p)
}
}
bool
isFinalizable(Thread* t, object o)
{
return t->m->heap->status(o) == Heap::Unreachable
and (classVmFlags
(t, static_cast<object>(t->m->heap->follow(objectClass(t, o))))
& HasFinalizerFlag);
}
void
clearTargetIfFinalizable(Thread* t, object r)
{
if (isFinalizable
(t, static_cast<object>(t->m->heap->follow(jreferenceTarget(t, r)))))
{
jreferenceTarget(t, r) = 0;
}
}
void
postVisit(Thread* t, Heap::Visitor* v)
{
@ -482,6 +501,30 @@ postVisit(Thread* t, Heap::Visitor* v)
assert(t, m->finalizeQueue == 0);
m->heap->postVisit();
for (object p = m->weakReferences; p;) {
object r = static_cast<object>(m->heap->follow(p));
p = jreferenceVmNext(t, r);
clearTargetIfFinalizable(t, r);
}
if (major) {
for (object p = m->tenuredWeakReferences; p;) {
object r = static_cast<object>(m->heap->follow(p));
p = jreferenceVmNext(t, r);
clearTargetIfFinalizable(t, r);
}
}
for (Reference* r = m->jniReferences; r; r = r->next) {
if (r->weak and isFinalizable
(t, static_cast<object>(t->m->heap->follow(r->target))))
{
r->target = 0;
}
}
object firstNewTenuredFinalizer = 0;
object lastNewTenuredFinalizer = 0;
@ -594,8 +637,6 @@ postVisit(Thread* t, Heap::Visitor* v)
m->tenuredWeakReferences = firstNewTenuredWeakReference;
}
m->heap->postVisit();
for (Reference* r = m->jniReferences; r; r = r->next) {
if (r->weak) {
if (m->heap->status(r->target) == Heap::Unreachable) {