Re: proposed smp_rendezvous change

On Monday 16 May 2011 17:54:54 Attilio Rao wrote:
> 2011/5/16 Max Laier <max_at_love2party.net>:
> > On Monday 16 May 2011 16:46:03 John Baldwin wrote:
> >> On Monday, May 16, 2011 4:30:44 pm Max Laier wrote:
> >> > On Monday 16 May 2011 14:21:27 John Baldwin wrote:
> >> > > Yes, we need to fix that.  Humm, it doesn't preempt when you do a
> >> > > critical_exit() though?  Or do you use a hand-rolled critical exit
> >> > > that doesn't do a deferred preemption?
> >> > 
> >> > Right now I just did a manual td_critnest++/--, but I guess ...
> >> 
> >> Ah, ok, so you would "lose" a preemption.  That's not really ideal.
> >> 
> >> > > Actually, I'm curious how the spin unlock inside the IPI could yield
> >> > > the CPU.  Oh, is rmlock doing a wakeup inside the IPI handler?  I
> >> > > guess that is ok as long as the critical_exit() just defers the
> >> > > preemption to the end of the IPI handler.
> >> > 
> >> > ... the earliest point where it is safe to preempt is after doing the
> >> > 
> >> >    atomic_add_int(&smp_rv_waiters[2], 1);
> >> > 
> >> > so that we can start other IPIs again.  However, since we don't accept
> >> > new IPIs until we signal EOI in the MD code (on amd64), this might
> >> > still not be a good place to do the yield?!?
> >> 
> >> Hmm, yeah, you would want to do the EOI before you yield.  However, we
> >> could actually move the EOI up before calling the MI code so long as we
> >> leave interrupts disabled for the duration of the handler (which we do).
> >> 
> >> > The spin unlock boils down to a critical_exit() and unless we did a
> >> > critical_enter() at some point during the redenvouz setup, we will
> >> > yield() if we owepreempt.  I'm not quite sure how that can happen, but
> >> > it seems like there is a path that allows the scheduler to set it from
> >> > a foreign CPU.
> >> 
> >> No, it is only set on curthread by curthread.  This is actually my main
> >> question.  I've no idea how this could happen unless the rmlock code is
> >> actually triggering a wakeup or sched_add() in its rendezvous handler.
> >> 
> >> I don't see anything in rm_cleanIPI() that would do that however.
> >> 
> >> I wonder if your original issue was really fixed  just by the first
> >> patch you had which fixed the race in smp_rendezvous()?
> > 
> > I found the stack that lead me to this patch in the first place:
> > 
> > #0  sched_switch (td=0xffffff011a970000, newtd=0xffffff006e6784b0,
> > flags=4) at src/sys/kern/sched_ule.c:1939
> > #1  0xffffffff80285c7f in mi_switch (flags=6, newtd=0x0) at
> > src/sys/kern/kern_synch.c:475
> > #2  0xffffffff802a2cb3 in critical_exit () at
> > src/sys/kern/kern_switch.c:185 #3  0xffffffff80465807 in spinlock_exit
> > () at
> > src/sys/amd64/amd64/machdep.c:1458
> > #4  0xffffffff8027adea in rm_cleanIPI (arg=<value optimized out>) at
> > src/sys/kern/kern_rmlock.c:180
> > #5  0xffffffff802b9887 in smp_rendezvous_action () at
> > src/sys/kern/subr_smp.c:402
> > #6  0xffffffff8045e2a4 in Xrendezvous () at
> > src/sys/amd64/amd64/apic_vector.S:235
> > #7  0xffffffff802a2c6e in critical_exit () at
> > src/sys/kern/kern_switch.c:179 #8  0xffffffff804365ba in uma_zfree_arg
> > (zone=0xffffff009ff4b5a0, item=0xffffff000f34cd40,
> > udata=0xffffff000f34ce08) at
> > src/sys/vm/uma_core.c:2370
> > .
> > .
> > .
> > 
> > and now that I look at it again, it is clear that critical_exit() just
> > isn't interrupt safe.  I'm not sure how to fix that, yet ... but this:
> > 
> > 
> >        if (td->td_critnest == 1) {
> >                td->td_critnest = 0;
> >                if (td->td_owepreempt) {
> >                        td->td_critnest = 1;
> > 
> > clearly doesn't work.
> 
> I'm sorry if I didn't reply to the whole rendezvous thread, but as
> long as there is so many people taking care of it, I'll stay hidden in
> my corner.
> 
> I just wanted to tell that I think you are misunderstanding what
> critical section is supposed to do.
> 
> When an interrupt fires, it goes on the old "interrupt/kernel context"
> which means it has not a context of his own. That is the reason why we
> disable interrupts on spinlocks (or similar workaround for !x86
> architectures) and this is why spinlocks are the only protection
> usable in code that runs in interrupt context.
> 
> Preempting just means another thread will be scheduler in the middle
> of another thread execution path.
> 
> This code is perfectly fine if you consider curthread won't be
> descheduled while it is executing.

Well, no - it is not.  With this you can end up with a curthread that has 
td_critnest=0 and td_owepreempt=1 in interrupt context.  If you use a spinlock 
on such a thread, it will do the preemption at the point where you drop the 
spinlock, this is bad in some circumstances.  One example is the smp_rendevous 
case we are discussing here.

Max