Using Dtrace for Performance Evaluation

I was looking at using dtrace to help characterize performance
for the new bxe(4) driver but I'm having problems with the very
simple task of capturing time spent in a function.  The D script
I'm using looks like the following:

#pragma D option quiet

fbt:if_bxe::entry
{
        self->in = timestamp;
}

fbt:if_bxe::return
{

        _at_callouts[((struct callout *)arg0)->c_func] = sum(timestamp -
            self->in);
}

tick-10sec
{
        printa("%40a %10_at_d\n", _at_callouts);
        clear(_at_callouts);
        printf("\n");
}

BEGIN
{
        printf("%40s | %s\n", "function", "nanoseconds per second");
}

After building dtrace into the kernel and loading the dtraceall
kernel module, when I load my bxe kernel module and run "dtrace -l"
to list all supported probes I notice that many functions have an 
entry probe but no exit probe.  This effectively prevents me from
calculating timestamps on "fbt:if_bxe::return" probes.  Why am I
seeing this behavior?

Dave