Re: Check in cpu_switchto triggering crash with PARANOIA on

[Nick Hudson <nick.hudson%gmx.co.uk@localhost>]

On 22/02/2021 04:15, Alan Fisher wrote:
> Hello,
>
> I've been trying to get the evbmips port working on a new chip recently,
> and in the process I've tried building the kernel with PARANOIA enabled.
> This has resulted in a crash on startup, and I am wondering if it is
> surfacing a bug. Here is what's happening:
>
> Some code under an #ifdef PARANOIA in cpu_switchto checks whether the
> IPL is IPL_SCHED, and if not, throws a trap. According to the manpage
> for cpu_switchto(9), the current IPL level being IPL_SCHED is a
> precondition for cpu_switchto(), so this check seems to make sense. The
> callstack looks like this:
>
> cpu_switchto  - this causes a trap when the check fails - manpage says
> IPL must be IPL_SCHED
> mi_switch - manpage says IPL must be IPL_SCHED
> yield - manpage doesn't say anything about IPL_SCHED, and IPL is not
> changed in this routine


    276 yield(void)
    277 {
    278 	struct lwp *l = curlwp;
    279
    280 	KERNEL_UNLOCK_ALL(l, &l->l_biglocks);
    281 	lwp_lock(l);

lwp_lock will raise the IPL to IPL_SCHED. spc_{lock,mutex} are used by
lwp_lock (maybe others)
HTH,

Nick