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

[Paul E. McKenney]

On Wed, May 20, 2020 at 09:51:17AM -0700, Andy Lutomirski wrote:
> 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?

>From an RCU viewpoint, yes, give or take my concerns about someone
putting rcu_tickle() on entry and rcu_irq_exit() on exit.  Perhaps
I can bring some lockdep trickery to bear.

But I must defer to Thomas and Peter on the non-RCU/non-NO_HZ_FULL
portions of this.

							Thanx, Paul