linux-kernel - Re: [PATCH RFC] x86/entry: Ask RCU if it needs rcu_irq

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Message-ID: <20200612135708.GJ4455@paulmck-ThinkPad-P72>
Date:   Fri, 12 Jun 2020 06:57:08 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org, luto@...nel.org,
        x86@...nel.org, frederic@...nel.org, rostedt@...dmis.org,
        joel@...lfernandes.org, mathieu.desnoyers@...icios.com,
        will@...nel.org, peterz@...radead.org
Subject: Re: [PATCH RFC] x86/entry: Ask RCU if it needs rcu_irq_{enter,exit}()

On Fri, Jun 12, 2020 at 11:27:21AM +0200, Thomas Gleixner wrote:
> "Paul E. McKenney" <paulmck@...nel.org> writes:
> > +static __always_inline bool rcu_needs_irq_enter(void)
> > +{
> > +	return !IS_ENABLED(CONFIG_TINY_RCU) &&
> > +               (context_tracking_enabled_cpu(smp_processor_id()) || is_idle_task(current));
> 
> This reintroduces the #PF problem which started the whole conditional
> RCU entry discussion:
> 
>    https://lore.kernel.org/lkml/20200515235125.628629605@linutronix.de/
> 
> and which made us all come to the conclusion that we can do it always
> conditional. No biscuit for you. :)

We can only be thankful that source-code control systems mean that my
coding session yesterday afternoon will have no permanent effect.

Let this be a lesson to all of you:  Hacking RCU while sleep-deprived
is a really bad idea.  ;-)

							Thanx, Paul