schedcpu() in /sys/kern/sched_4bsd.c calls thread_lock() on thread with un-initialized td_lock

[John Baldwin]

On Thursday, March 31, 2011 12:34:31 pm Attilio Rao wrote:
> 2011/3/31 John Baldwin <jhb at freebsd.org>:
> > On Thursday, March 31, 2011 7:32:26 am Svatopluk Kraus wrote:
> >> Hi,
> >>
> >>   I've got a page fault (because of NULL td_lock) in
> >> thread_lock_flags() called from schedcpu() in /sys/kern/sched_4bsd.c
> >> file. During process fork, new thread is linked to new process which
> >> is linked to allproc list and both allproc_lock and new process lock
> >> are unlocked before sched_fork() is called, where new thread td_lock
> >> is initialized. Only PRS_NEW process status is on sentry but not
> >> checked in schedcpu().
> >
> > I think this should fix it:
> >
> > Index: sched_4bsd.c
> > ===================================================================
> > --- sched_4bsd.c        (revision 220190)
> > +++ sched_4bsd.c        (working copy)
> > @@ -463,6 +463,10 @@ schedcpu(void)
> >        sx_slock(&allproc_lock);
> >        FOREACH_PROC_IN_SYSTEM(p) {
> >                PROC_LOCK(p);
> > +               if (p->p_state == PRS_NEW) {
> > +                       PROC_UNLOCK(p);
> > +                       continue;
> > +               }
> >                FOREACH_THREAD_IN_PROC(p, td) {
> >                        awake = 0;
> >                        thread_lock(td);
> >
> 
> I don't really think this fix is right because otherwise, when using
> sched_4bsd anytime we are going to scan the thread list within a proc
> we need to check for PRS_NEW.
> 
> We likely need to change the init scheme for the td_lock by having a
> scheduler primitive setting it and doing that on thread_init() UMA
> constructor, or similar approach.

But the thread state isn't valid anyway.  4BSD shouldn't be touching the 
thread since it is in an incomplete / undefined state.

-- 
John Baldwin