Question WRT early IRQ/NMI entry code

[Nicolas Saenz Julienne]

On Tue, 2021-11-30 at 14:47 +0100, Thomas Gleixner wrote:
> On Tue, Nov 30 2021 at 12:28, Nicolas Saenz Julienne wrote:
> > while going over the IRQ/NMI entry code I've found a small 'inconsistency':
> > while in the IRQ entry path, we inform RCU of the context change *before*
> > incrementing the preempt counter, the opposite happens for the NMI entry
> > path. This applies to both arm64 and x86[1].
> > 
> > Actually, rcu_nmi_enter() — which is also the main RCU context switch function
> > for the IRQ entry path — uses the preempt counter to verify it's not in NMI
> > context. So it would make sense to assume all callers have the same updated
> > view of the preempt count, which isn't true ATM.
> > 
> > I'm sure there an obscure/non-obvious reason for this, right?
> 
> There is.
> 
> > IRQ path:
> >   -> x86_64 asm (entry_64.S)
> >   -> irqentry_enter() -> rcu_irq_enter() -> *rcu_nmi_enter()*
> >   -> run_irq_on_irqstack_cond() -> irq_exit_rcu() -> *preempt_count_add(HARDIRQ_OFFSET)*
> >   -> // Run IRQ...
> > 
> > NMI path:
> >   -> x86_64 asm (entry_64.S)
> >   -> irqentry_nmi_enter() -> __nmi_enter() -> *__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET)*
> >                           -> *rcu_nmi_enter()*
> 
> The reason is symmetry vs. returning from interupt / exception:
> 
>  irqentry_enter()
>       exit_rcu = false;
> 
>       if (user_mode(regs)) {
>           irqentry_enter_from_user_mode(regs)
>             __enter_from_user_mode(regs)
>               user_exit_irqoff();       <- RCU handling for NOHZ full
> 
>       } else if (is_idle_task_current()) {
>             rcu_irq_enter()
>             exit_rcu = true;
>       }
> 
>  irq_enter_rcu()
>      __irq_enter_raw()
>      preempt_count_add(HARDIRQ_OFFSET);
> 
>  irq_handler()
> 
>  irq_exit_rcu()
>      preempt_count_sub(HARDIRQ_OFFSET);
>      if (!in_interrupt() && local_softirq_pending())
>      	 invoke_softirq();
> 
>  irqentry_exit(regs, exit_rcu)
> 
>      if (user_mode(regs)) {
>          irqentry_exit_to_usermode(regs)
>            user_enter_irqoff();     <- RCU handling for NOHZ full
>      } else if (irqs_enabled(regs)) {
>            if (exit_rcu) {          <- Idle task special case
>                rcu_irq_exit();
>            } else {
>               irqentry_exit_cond_resched();
>            }
> 
>      } else if (exit_rcu) {
>          rcu_irq_exit();
>      }
> 
> On return from interrupt HARDIRQ_OFFSET has to be removed _before_
> handling soft interrupts. It's also required that the preempt count has
> the original state _before_ reaching irqentry_exit() which
> might schedule if the interrupt/exception hit user space or kernel space
> with interrupts enabled.
> 
> So doing it symmetric makes sense.
> 
> For NMIs the above conditionals do not apply at all and we just do
> 
>     __nmi_enter()
>         preempt_count_add(NMI_COUNT + HARDIRQ_COUNT);
>     rcu_nmi_enter();
> 
>     handle_nmi();
> 
>     rcu_nmi_exit();
>     __nmi_exit()
>         preempt_count_sub(NMI_COUNT + HARDIRQ_COUNT);
> 
> The reason why preempt count is incremented before invoking
> rcu_nmi_enter() is simply that RCU has to know about being in NMI
> context, i.e. in_nmi() has to return the correct answer.
> 
> Thanks,

Thanks Thomas!

-- 
Nicolás Sáenz