cvs commit: src/sys/kern kern_mutex.c

[John Baldwin]

On Tuesday 05 June 2007 11:43:03 am Attilio Rao wrote:
> 2007/6/5, Attilio Rao <attilio at freebsd.org>:
> > 2007/6/5, Bruce Evans <brde at optusnet.com.au>:
> > >
> > > I get a "spin lock held too long" panic during (an interrupt in?) acpi
> > > initialization on booting non-PREEMPTION SCHED_4BSD SMP.  Haven't tried
> > > other cases.
> >
> > Do you have a backtrace or any other debugging stuffs available?
> 
> Mmm, I think I got the bug.
> basically, in kern_mutex.c::_mtx_unlock_sleep(), in the not-preemptive
> case what happens at some point is:
> 
> td = curthread;
> if (td->td_critnest > 0 || td1->td_priority >= td->td_priority)
>          return;
> 
> thread_lock(td1);
> if (!TD_IS_RUNNING(td1)) {
> ...
> 
> mi_switch(SW_INVOL, NULL);
> ...
> }
> thread_unlock(td1);
> 
> Which is wrong beacause td1 is not curthread and really curthread
> should be locked too when context switching.
> 
> To a first look the idea is that td and td1 should be locked both, but
> I just want more time to look better at it.

If this is the old #ifndef PREEMPTION manual preemption stuff, then just 
remove it.  I've been wanting to axe it for a while, rwlocks don't do the 
manual preemption either, and if it is getting in the way it's best to just 
purge it.

-- 
John Baldwin