TBB task allocation assertion - c++

Question

I'm trying to traverse a tree via TBB tasks and continuations. The code is below. When I run the code it keeps aborting (frequently, although not always) with the following error:
Assertion t_next->state()==task::allocated failed on line 334 of file ../../src/tbb/custom_scheduler.h
Detailed description: if task::execute() returns task, it must be marked as allocated
What can be causing this problem?
template<class NodeVisitor>
void
traverse_tree(NodeVisitor& nv)
{
TreeTraversal<NodeVisitor>& tt = *(new(task::allocate_root()) TreeTraversal<NodeVisitor>(nv));
task::spawn_root_and_wait(tt);
}
template<class NodeVisitor>
class TreeTraversal: public task
{
public:
struct Continuation;
public:
TreeTraversal(NodeVisitor nv_):
nv(nv_) {}
task* execute()
{
nv.pre();
Continuation* c = new(allocate_continuation()) Continuation(nv);
c->set_ref_count(nv.size());
for (size_t i = 0; i < nv.size(); ++i)
{
TreeTraversal& tt = *(new(c->allocate_child()) TreeTraversal(nv.child(i)));
spawn(tt);
}
if (!nv.size())
return c;
return NULL;
}
private:
NodeVisitor nv;
};
template<class NodeVisitor>
class TreeTraversal<NodeVisitor>::Continuation: public task
{
public:
Continuation(NodeVisitor& nv_):
nv(nv_) {}
task* execute() { nv.post(); return NULL; }
private:
NodeVisitor nv;
};

Answer 1

I have never seen before that a task is allocated as a continuation and then returned from execute(). That might be the reason of the assertion failure (update: an experiment showed it is not, see details below).
Meanwhile, you can change the code of TreeTraversal::execute() to be roughly this:
nv.pre();
if (!nv.size())
nv.post();
else {
// Do all the task manipulations
}
return NULL;
Update: a simplified test shown below worked well on my dual-core laptop. That makes me suppose possible memory corruption in your actual code, in which case the re-shuffling suggested above might just hide the issue but not fix it.
#include "tbb/task.h"
using namespace tbb;
class T: public task {
public:
class Continuation: public task {
public:
Continuation() {}
task* execute() { return NULL; }
};
private:
size_t nv;
public:
T(size_t n): nv(n) {}
task* execute() {
Continuation* c = new(allocate_continuation()) Continuation();
c->set_ref_count(nv);
for (size_t i = 0; i < nv; ++i) {
T& tt = *(new(c->allocate_child()) T(nv-i-1));
spawn(tt);
}
return (nv==0)? c : NULL;
}
};
int main() {
T& t = *new( task::allocate_root() ) T(24);
task::spawn_root_and_wait(t);
return 0;
}