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

[Thomas Gleixner]

Andy Lutomirski <luto@xxxxxxxxxx> writes:
> 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.

I thought about that, but then ended up with the conclusion that RCU
might be unhappy, but my conclusion might be fundamentally wrong.

Paul?

Thanks,

        tglx