Re: proposed smp_rendezvous change

2011/5/17 Max Laier <max_at_love2party.net>:
> 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.

This circumstances are further protected by another call to
critical_enter(), by consumers or however upper layer calls.
This is why, for example, spinlock_enter() does call critical_enter()
even if it actually disables interrupts or why we disable preemption
in other cases where the interrupts are already disabled.

Attilio


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