On Tue, 30 Nov 2021 14:47:01 +0100
Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 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.
Seems like there should be a comment in the code somewhere that explains
this.
-- Steve
