linux-kernel - Re: Question WRT early IRQ/NMI entry code

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Message-ID: <20211130091356.7336e277@gandalf.local.home>
Date:   Tue, 30 Nov 2021 09:13:56 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        rcu@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>, paulmck@...nel.org,
        mtosatti <mtosatti@...hat.com>, frederic <frederic@...nel.org>
Subject: Re: Question WRT early IRQ/NMI entry code

On Tue, 30 Nov 2021 14:47:01 +0100
Thomas Gleixner <tglx@...utronix.de> 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