linux-kernel - Re: [PATCH x86/entry: Force rcu_irq

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Message-ID: <871rmfcskb.fsf@nanos.tec.linutronix.de>
Date:   Tue, 16 Jun 2020 10:40:04 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org,
        Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task

Joel,

Joel Fernandes <joel@...lfernandes.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