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

2011/3/31 John Baldwin <jhb_at_freebsd.org>:
> On Thursday, March 31, 2011 2:20:11 pm Attilio Rao wrote:
>> 2011/3/31 John Baldwin <jhb_at_freebsd.org>:
>> > 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)
>> >> > _at__at_ -463,6 +463,10 _at__at_ 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.
>>
>> Yep, in this case I'd then want to just add the threads to proc once
>> they are fully initialized.
>>
>> It is pointless (and dangerous) to replicate this check all over,
>> besides we want scheduler agnostic code, which means every iterations
>> of p_threads will need to check for a valid state of threads.
>
> Yes, we do have to check for PRS_NEW in many places with the current approach,
> but we need some way to reserve the PID to avoid duplicates and unless we
> expand the scope of allproc in fork by a whole lot or stop using the allproc
> list to track "pids in use", we will be stuck with some sort of "process
> is still being built" sentry.

Yes, you are right, I was assuming you wanted to work on a larger
patchset though.
If you are happy enough with the band-aid, for the moment, ok, but I
strongly raccomand to change this in the future (could be a nice task
to work through BSDCan, for example).

Attilio


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