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

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Date:   Tue, 30 Nov 2021 12:28:41 +0100
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        rcu@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Steven Rostedt <rostedt@...dmis.org>, paulmck@...nel.org,
        mtosatti <mtosatti@...hat.com>, frederic <frederic@...nel.org>
Subject: Question WRT early IRQ/NMI entry code

Hi All,
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?

Thanks!
Nicolas

[1] 
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()*

For arm64, see 'arch/arm64/kernel/entry-common.c'.