Re: [PATCH 06/10] x86/entry: Move nmi entry/exit into common code

Thomas Gleixner
2020-10-27T18:03:58Z

On Tue, Oct 27 2020 at 00:07, Ira Weiny wrote:
> On Fri, Oct 23, 2020 at 11:50:11PM +0200, Thomas Gleixner wrote:
>> > #ifndef irqentry_state
>> > typedef struct irqentry_state {
>> > - bool exit_rcu;
>> > + union {
>> > + bool exit_rcu;
>> > + bool lockdep;
>> > + };
>> > } irqentry_state_t;
>> > #endif
>>
>> -E_NO_KERNELDOC
>
> Adding: Paul McKenney
>
> I'm happy to write something but I'm very unfamiliar with this code. So I'm
> getting confused what exactly exit_rcu is flagging.
>
> I can see that exit_rcu is a bad name for the state used in
> irqentry_nmi_[enter|exit](). Furthermore, I see why 'lockdep' is a better
> name. But similar lockdep handling is used in irqentry_exit() if exit_rcu is
> true...

No, it's not similar at all. Lockdep state vs. interrupts and regular
exceptions is always consistent.

In the NMI case, that's not guaranteed because of

local_irq_disable()
arch_local_irq_disable()
<- NMI race window
trace_hardirqs_off()

same the other way round

local_irq_enable()
trace_hardirqs_on()
<- NMI race window
arch_local_irq_enable()

IOW, the hardware state and the lockdep state are not consistent.

> /**
> * struct irqentry_state - Opaque object for exception state storage
> * @exit_rcu: Used exclusively in the irqentry_*() calls; tracks if the
> * exception hit the idle task which requires special handling,
> * including calling rcu_irq_exit(), when the exception
> exits.

calls; signals whether the exit path has to invoke rcu_irq_exit().

> * @lockdep: Used exclusively in the irqentry_nmi_*() calls; ensures lockdep
> * tracking is maintained if hardirqs were already enabled

ensures that lockdep state is restored correctly on exit from nmi.

> *
> * This opaque object is filled in by the irqentry_*_enter() functions and
> * should be passed back into the corresponding irqentry_*_exit()
> functions

s/should/must/

> * when the exception is complete.
> *
> * Callers of irqentry_*_[enter|exit]() should consider this structure
> opaque

s/should/must/

> * and all members private. Descriptions of the members are provided to aid in
> * the maintenance of the irqentry_*() functions.
> */
>
> Perhaps Paul can enlighten me on how exit_rcu is used beyond just flagging a
> call to rcu_irq_exit()?

I can do that as well :) The only purpose is to invoke rcu_irq_exit()
conditionally.

> Why do we call lockdep_hardirqs_off() only when in the idle task? That implies
> that regs_irqs_disabled() can only be false if we were in the idle task to
> match up the lockdep on/off calls.

You're reading the code slightly wrong.

> This does not make sense to me because why do we need the extra check
> for exit_rcu? I'm still trying to understand when regs_irqs_disabled() is false.

It's false when the interrupted context had interrupts enabled.

So we have the following scenarios:

Usermode Idletask irqs enabled RCU entry RCU exit
Y N Y Y Y

N N Y N N
N N N N N
N Y Y Y Y
N Y N Y Y

Now you might wonder about irqs enabled/disabled. This code is not only
used for interrupts (device, ipi, local timer...) where interrupts are
obviously enabled, it's also used for exception entry/exit. You can have
e.g. pagefaults in interrupt disabled regions.

> Also, the comment in irqentry_enter() refers to irq_enter_from_user_mode() which
> does not seem to exist anymore. So I'm not sure what careful sequence it is
> referring to.

That was renamed to irqentry_enter_from_user_mode() and the comment was
not updated. Sorry for leaving this hard to solve puzzle around.

Thanks,

tglx