Question WRT early IRQ/NMI entry code

[Frederic Weisbecker]

On Tue, Nov 30, 2021 at 12:28:41PM +0100, Nicolas Saenz Julienne wrote:
> 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?

At least I can't find an immediate reason for it either :-)

Thanks!