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

[Andy Lutomirski]

On Wed, May 20, 2020 at 8:36 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Tue, May 19, 2020 at 7:23 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Tue, May 19, 2020 at 05:26:58PM -0700, Andy Lutomirski wrote:
> > > On Tue, May 19, 2020 at 2:20 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

First, the patch as you submitted it is Acked-by: Andy Lutomirski
<luto@xxxxxxxxxx>.  I think there are cleanups that should happen, but
I think the patch is correct.

About cleanups, concretely:  I think that everything that calls
__idtenter_entry() is called in one of a small number of relatively
sane states:

1. User mode.  This is easy.

2. Kernel, RCU is watching, everything is sane.  We don't actually
need to do any RCU entry/exit pairs -- we should be okay with just a
hypothetical RCU tickle (and IRQ tracing, etc).  This variant can
sleep after the entry part finishes if regs->flags & IF and no one
turned off preemption.

3. Kernel, RCU is not watching, system was idle.  This can only be an
actual interrupt.

So maybe the code can change to:

    if (user_mode(regs)) {
        enter_from_user_mode();
    } else {
        if (!__rcu_is_watching()) {
            /*
             * If RCU is not watching then the same careful
             * sequence vs. lockdep and tracing is required.
             *
             * This only happens for IRQs that hit the idle loop, and
             * even that only happens if we aren't using the sane
             * MWAIT-while-IF=0 mode.
             */
            lockdep_hardirqs_off(CALLER_ADDR0);
            rcu_irq_enter();
            instrumentation_begin();
            trace_hardirqs_off_prepare();
            instrumentation_end();
            return true;
        } else {
            /*
             * If RCU is watching then the combo function
             * can be used.
             */
            instrumentation_begin();
            trace_hardirqs_off();
            rcu_tickle();
            instrumentation_end();
        }
    }
    return false;

This is exactly what you have except that the cond_rcu part is gone
and I added rcu_tickle().

Paul, the major change here is that if an IRQ hits normal kernel code
(i.e. code where RCU is watching and we're not in an EQS), the IRQ
won't call rcu_irq_enter() and rcu_irq_exit().  Instead it will call
rcu_tickle() on entry and nothing on exit.  Does that cover all the
bases?