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

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Date:	Mon, 11 Jun 2012 14:55:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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 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.

							Thanx, Paul

> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  kernel/rcutree.c |   19 ++++++++-----------
>  1 files changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 1b0dca2..7a54265 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -619,12 +619,12 @@ void rcu_user_exit(void)
> 
>  /**
>   * rcu_user_exit_irq - inform RCU that we won't resume to userspace
> - * idle mode after the current irq returns.
> + * idle mode after the current non-nesting irq returns.
>   *
> - * This is similar to rcu_user_exit() but in the context of a non
> - * nesting irq. This is called when the irq has interrupted a userspace
> - * RCU idle mode context. When the interrupt returns after this call,
> - * the CPU won't restore the RCU idle mode.
> + * This is similar to rcu_user_exit() but in the context of an
> + * irq. This is called when the irq has interrupted a userspace
> + * RCU idle mode context. When the current non-nesting interrupt
> + * returns after this call, the CPU won't restore the RCU idle mode.
>   */
>  void rcu_user_exit_irq(void)
>  {
> @@ -633,12 +633,9 @@ void rcu_user_exit_irq(void)
> 
>  	local_irq_save(flags);
>  	rdtp = &__get_cpu_var(rcu_dynticks);
> -	/*
> -	 * Ensure this irq is a non-nesting one interrupting
> -	 * an RCU idle mode.
> -	 */
> -	WARN_ON_ONCE(rdtp->dynticks_nesting != 1);
> -	rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE + 1;
> +	/* Ensure we are interrupting an RCU idle mode. */
> +	WARN_ON_ONCE(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK);
> +	rdtp->dynticks_nesting += DYNTICK_TASK_EXIT_IDLE;
>  	local_irq_restore(flags);
>  }
> 
> -- 
> 1.7.0.4
> 
> --
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