linux-kernel - Re: [PATCH] rcu: Allow calls to rcu_exit_user

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Date:	Sun, 10 Jun 2012 22:29:17 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] rcu: Allow calls to rcu_exit_user_irq from nesting irqs

On Sun, Jun 10, 2012 at 11:06:59AM -0700, Paul E. McKenney wrote:
> On Sun, Jun 10, 2012 at 12:55:54AM +0200, Frederic Weisbecker wrote:
> >  void rcu_user_enter_irq(void)
> >  {
> > @@ -478,12 +478,9 @@ void rcu_user_enter_irq(void)
> > 
> >  	local_irq_save(flags);
> >  	rdtp = &__get_cpu_var(rcu_dynticks);
> > -	/*
> > -	 * Ensure this irq is a non nesting one interrupting
> > -	 * a non-idle RCU state.
> > -	 */
> > -	WARN_ON_ONCE(rdtp->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE + 1);
> > -	rdtp->dynticks_nesting = 1;
> > +	/* Ensure we are interrupting a non-idle RCU state */
> > +	WARN_ON_ONCE(!(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK));
> > +	rdtp->dynticks_nesting -= DYNTICK_TASK_EXIT_IDLE;
> 
> This will be broken on architectures that can fail to return from interrupts
> and exceptions and vice versa.  The resulting value of rdtp->dynticks_nesting
> might well go negative, or might fail to reach zero when the outermost
> interrupt returns.
> 
> One workaround would be to add up the relevant fields of preempt_count()
> and assign the result to rdtp->dynticks_nesting.

That's ok. I made rcu_user_enter_irq() to support nesting irqs in order to stay
consistant with the same change in rcu_user_exit_irq(). But this is not necessary.
rcu_user_enter_irq() itself is only called from the outermost irq.

Also there is a sanity check that ensures that in the current code in rcu/idle tree
so rcu_user_enter_irq() can stay as is there.

I'll resend the patch with only the change in rcu_user_exit_irq().

Thanks.
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