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

[Thomas Gleixner]

Andy Lutomirski <luto@xxxxxxxxxx> writes:
> On Wed, May 20, 2020 at 8:36 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>     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?

Just chatted with Paul on IRC and he thinks this should work, but he's
not sure whether it's actually sane :)

Thanks,

        tglx