linux-kernel - Re: [PATCH v2] rcu: Allow calls to rcu_exit_user

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Date:	Tue, 12 Jun 2012 00:06:04 +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 v2] rcu: Allow calls to rcu_exit_user_irq from nesting
 irqs

On Mon, Jun 11, 2012 at 02:55:46PM -0700, Paul E. McKenney wrote:
> On Sun, Jun 10, 2012 at 11:47:26PM +0200, Frederic Weisbecker wrote:
> > From: Frederic Weisbecker <fweisbec@...il.com>
> > Date: Sat, 9 Jun 2012 14:06:30 +0200
> > Subject: [PATCH] rcu: Allow calls to rcu_exit_user_irq from nesting irqs
> > 
> > rcu_exit_user_irq() which exits RCU idle mode after the current
> > irq returns has been designed to be called from non nesting irqs
> > only.
> > 
> > However the IPI that restarts the tick and exits RCU user-idle mode
> > in nohz cpusets can happen anytime. For example it can be a nesting
> > irq by interrupting a softirq. In this case the stack of RCU API
> > calls becomes:
> > 
> > ==> IRQ
> >     rcu_irq_enter()
> >     ....
> >     do_softirq {
> > ===== > IRQ (restart tick IPI)
> >         rcu_irq_enter()
> >         rcu_exit_user_irq()
> >         rcu_irq_exit()
> > <=====
> >     }
> >     rcu_irq_exit();
> > 
> > Hence we need to make rcu_exit_user_irq() callable from any nesting
> > level of interrupt.
> > 
> > v2: rcu_user_enter_irq() is only called from non nesting irqs though so
> > the change doesn't need to be propagated to it.
> 
> I have queued this on -rcu on branch rcu/idle.  I have also removed
> the TINY_RCU commits as discussed.  If you ever need them back, the
> magic branch name is idle.2012.06.06a.

Thanks a lot!
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