Re: [PATCHv2 1/5] arm64/entry-common: push the judgement of nmi ahead

[Pingfan Liu]

On Fri, Oct 08, 2021 at 12:01:25PM +0800, Pingfan Liu wrote:
> Sorry that I missed this message and I am just back from a long
> festival.
> 
> Adding Paul for RCU guidance.
> 
> On Thu, Sep 30, 2021 at 02:32:57PM +0100, Mark Rutland wrote:
> > On Sat, Sep 25, 2021 at 11:39:55PM +0800, Pingfan Liu wrote:
> > > On Fri, Sep 24, 2021 at 06:53:06PM +0100, Mark Rutland wrote:
> > > > On Fri, Sep 24, 2021 at 09:28:33PM +0800, Pingfan Liu wrote:
> > > > > In enter_el1_irq_or_nmi(), it can be the case which NMI interrupts an
> > > > > irq, which makes the condition !interrupts_enabled(regs) fail to detect
> > > > > the NMI. This will cause a mistaken account for irq.
> > > > 
> > > Sorry about the confusing word "account", it should be "lockdep/rcu/.."
> > > 
> > > > Can you please explain this in more detail? It's not clear which
> > > > specific case you mean when you say "NMI interrupts an irq", as that
> > > > could mean a number of distinct scenarios.
> > > > 
> > > > AFAICT, if we're in an IRQ handler (with NMIs unmasked), and an NMI
> > > > causes a new exception we'll do the right thing. So either I'm missing a
> > > > subtlety or you're describing a different scenario..
> > > > 
> > > > Note that the entry code is only trying to distinguish between:
> > > > 
> > > > a) This exception is *definitely* an NMI (because regular interrupts
> > > >    were masked).
> > > > 
> > > > b) This exception is *either* and IRQ or an NMI (and this *cannot* be
> > > >    distinguished until we acknowledge the interrupt), so we treat it as
> > > >    an IRQ for now.
> > > > 
> > > b) is the aim.
> > > 
> > > At the entry, enter_el1_irq_or_nmi() -> enter_from_kernel_mode()->rcu_irq_enter()/rcu_irq_enter_check_tick() etc.
> > > While at irqchip level, gic_handle_irq()->gic_handle_nmi()->nmi_enter(),
> > > which does not call rcu_irq_enter_check_tick(). So it is not proper to
> > > "treat it as an IRQ for now"
> > 
> > I'm struggling to understand the problem here. What is "not proper", and
> > why?
> > 
> > Do you think there's a correctness problem, or that we're doing more
> > work than necessary? 
> > 
> I had thought it just did redundant accounting. But after revisiting RCU
> code, I think it confronts a real bug.
> 
> > If you could give a specific example of a problem, it would really help.
> > 
> Refer to rcu_nmi_enter(), which can be called by
> enter_from_kernel_mode():
> 
> ||noinstr void rcu_nmi_enter(void)
> ||{
> ||        ...
> ||        if (rcu_dynticks_curr_cpu_in_eqs()) {
> ||
> ||                if (!in_nmi())
> ||                        rcu_dynticks_task_exit();
> ||
> ||                // RCU is not watching here ...
> ||                rcu_dynticks_eqs_exit();
> ||                // ... but is watching here.
> ||
> ||                if (!in_nmi()) {
> ||                        instrumentation_begin();
> ||                        rcu_cleanup_after_idle();
> ||                        instrumentation_end();
> ||                }
> ||
> ||                instrumentation_begin();
> ||                // instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
> ||                instrument_atomic_read(&rdp->dynticks, sizeof(rdp->dynticks));
> ||                // instrumentation for the noinstr rcu_dynticks_eqs_exit()
> ||                instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> ||
> ||                incby = 1;
> ||        } else if (!in_nmi()) {
> ||                instrumentation_begin();
> ||                rcu_irq_enter_check_tick();
> ||        } else  {
> ||                instrumentation_begin();
> ||        }
> ||        ...
> ||}
> 

Forget to supplement the context for understanding the case:
  On arm64, at present, a pNMI (akin to NMI) may call rcu_nmi_enter()
  without calling "__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);".
  As a result it can be mistaken as an normal interrupt in
  rcu_nmi_enter().

And this may cause the following issue:
> There is 3 pieces of code put under the
> protection of if (!in_nmi()). At least the last one
> "rcu_irq_enter_check_tick()" can trigger a hard lock up bug. Because it
> is supposed to hold a spin lock with irqoff by
> "raw_spin_lock_rcu_node(rdp->mynode)", but pNMI can breach it. The same
> scenario in rcu_nmi_exit()->rcu_prepare_for_idle().
> 
> As for the first two "if (!in_nmi())", I have no idea of why, except
> breaching spin_lock_irq() by NMI. Hope Paul can give some guide.
> 
> 
> Thanks,
> 
> 	Pingfan
> 
> 
> > I'm aware that we do more work than strictly necessary when we take a
> > pNMI from a context with IRQs enabled, but that's how we'd intended this
> > to work, as it's vastly simpler to manage the state that way. Unless
> > there's a real problem with that approach I'd prefer to leave it as-is.
> > 
> > Thanks,
> > Mark.
> > 
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