Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task

[Thomas Gleixner <tglx@linutronix.de>]

Joel,

Joel Fernandes <joel@joelfernandes.org> writes:
> On Fri, Jun 12, 2020 at 03:55:00PM +0200, Thomas Gleixner wrote:
>> This is also correct vs. user mode entries in NOHZ full scenarios because
>> user mode entries bring RCU out of EQS and force the RCU irq nesting state
>
> I had to re-read this sentence a couple of times. The 'user mode entries'
> sounds like 'entry into user mode'. It would be good to reword it to 'IRQ
> entry in user mode'.

:)

> My knowledge predates the rcu-watching reworks so apologies on the below
> question but I still didn't fully follow why when the idle task behaves
> differently from being in user mode. Even with user mode we should have:
>
> <user mode>  (in EQS)
>   <irq entry>   <---------- exits EQS so now rcu is watching
>      <softirq entry in the exit path>
>         <timer tick irq entry>  <-- the buggy !watching logic prevents rcu_irq_enter
> 	    -> report QS since tick thinks it is first level.
>
> Is there a subtlety here I'm missing? I checked the RCU code and I did not
> see anywhere that rcu_user_enter() makes it behave differently. Both
> rcu_user_enter() and rcu_idle_enter() call rcu_eqs_enter().

The interrupt hit user mode entry does:

    idtentry_enter_cond_rcu()
      if (user_mode(regs))
         enter_from_user_mode()
           user_exit_irqoff()
             __context_tracking_exit(CONTEXT_USER)
               rcu_user_exit()
                 rcu_eqs_exit(1)
                   ...
                   WRITE_ONCE(rdp->dynticks_nmi_nesting,
                              DYNTICK_IRQ_NONIDLE);

i.e. for interrupts which enter from user mode we are not invoking
rcu_irq_enter() at all.

The return from interrupt path has nothing to do with that because
that's an entry in kernel mode.

Thanks,

        tglx