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:

quoted

On Fri, Sep 24, 2021 at 06:53:06PM +0100, Mark Rutland wrote:

quoted

On Fri, Sep 24, 2021 at 09:28:33PM +0800, Pingfan Liu wrote:

quoted

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/.."

quoted

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();
||        }
||        ...
||}

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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