Is there any reason that a Critical Input Interrupt will not work reliably on a 44x powerpc?

I am using an AMCC now Applied Micro AMCC460EX

and changed

CRITICAL_EXCEPTION(0x0100, CriticalInput, unknown_exception)
to
CRITICAL_EXCEPTION(0x0100, CriticalInput, do_MyCritIntr)

The code for the handler is trivial
ie.
int crintrcount;

void do_MyCritIntr(void)
{
  crintrcount++;
}

The code runs for a while but eventually I get panic messages and  the system hangs.  Is there something I must alter in the kernel to do this reliably?

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