Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu()

[Andy Lutomirski]

On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
>
> The pagefault handler cannot use the regular idtentry_enter() because that
> invokes rcu_irq_enter() if the pagefault was caused in the kernel. Not a
> problem per se, but kernel side page faults can schedule which is not
> possible without invoking rcu_irq_exit().
>
> Adding rcu_irq_exit() and a matching rcu_irq_enter() into the actual
> pagefault handling code would be possible, but not pretty either.
>
> Provide idtentry_entry/exit_cond_rcu() which calls rcu_irq_enter() only
> when RCU is not watching. The conditional RCU enabling is a correctness
> issue: A kernel page fault which hits a RCU idle reason can neither
> schedule nor is it likely to survive. But avoiding RCU warnings or RCU side
> effects is at least increasing the chance for useful debug output.
>
> The function is also useful for implementing lightweight reschedule IPI and
> KVM posted interrupt IPI entry handling later.

Why is this conditional?  That is, couldn't we do this for all
idtentry_enter() calls instead of just for page faults?  Evil things
like NMI shouldn't go through this path at all.

--Andy