Re: proposed smp_rendezvous change

On Tuesday, May 17, 2011 12:20:40 pm Max Laier wrote:
> On 05/17/2011 05:16 AM, John Baldwin wrote:
> ...
> > Index: kern/kern_switch.c
> > ===================================================================
> > --- kern/kern_switch.c (revision 221536)
> > +++ kern/kern_switch.c (working copy)
> > _at__at_ -192,15 +192,22 _at__at_
> > critical_exit(void)
> > {
> > struct thread *td;
> > - int flags;
> > + int flags, owepreempt;
> >
> > td = curthread;
> > KASSERT(td->td_critnest != 0,
> > ("critical_exit: td_critnest == 0"));
> >
> > if (td->td_critnest == 1) {
> > + owepreempt = td->td_owepreempt;
> > + td->td_owepreempt = 0;
> > + /*
> > + * XXX: Should move compiler_memory_barrier() from
> > + * rmlock to a header.
> > + */
> 
> XXX: If we get an interrupt at this point and td_owepreempt was zero, 
> the new interrupt will re-set it, because td_critnest is still non-zero.
> 
> So we still end up with a thread that is leaking an owepreempt *and* 
> lose a preemption.

I don't see how this can still leak owepreempt.  The nested interrupt should 
do nothing (except for possibly set owepreempt) until td_critnest is 0.

However, we can certainly lose preemptions.

I wonder if we can abuse the high bit of td_critnest for the owepreempt flag 
so it is all stored in one cookie.  We only set owepreempt while holding 
thread_lock() (so interrupts are disabled), so I think we would be ok and not 
need atomic ops.

Hmm, actually, the top-half code would have to use atomic ops.  Nuts.  Let me 
think some more.

-- 
John Baldwin