linux-kernel - Re: [tip:timers/urgent] timers/tick/broadcast-hrtimer: Fix suspicious RCU usage in idle loop

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Date:	Thu, 26 Mar 2015 09:18:20 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	peterz@...radead.org, paulmck@...ux.vnet.ibm.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, mingo@...nel.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/urgent] timers/tick/broadcast-hrtimer:  Fix suspicious
 RCU usage in idle loop

Hi,

Can you please add stable@...r.kernel.org to the Cc?
Without the patch, we get RCU warnings during bootup.
Hence the patch is important for the stable kernels as well.

Regards
Preeti U Murthy

On 03/23/2015 05:54 PM, tip-bot for Preeti U Murthy wrote:
> Commit-ID:  a127d2bcf1fbc8c8e0b5cf0dab54f7d3ff50ce47
> Gitweb:     http://git.kernel.org/tip/a127d2bcf1fbc8c8e0b5cf0dab54f7d3ff50ce47
> Author:     Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> AuthorDate: Wed, 18 Mar 2015 16:19:27 +0530
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Mon, 23 Mar 2015 10:50:05 +0100
> 
> timers/tick/broadcast-hrtimer: Fix suspicious RCU usage in idle loop
> 
> The hrtimer mode of broadcast queues hrtimers in the idle entry
> path so as to wakeup cpus in deep idle states. The associated
> call graph is :
> 
> 	cpuidle_idle_call()
> 	|____ clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, ....))
> 	     |_____tick_broadcast_set_event()
> 		   |____clockevents_program_event()
> 			|____bc_set_next()
> 
> The hrtimer_{start/cancel} functions call into tracing which uses RCU.
> But it is not legal to call into RCU in cpuidle because it is one of the
> quiescent states. Hence protect this region with RCU_NONIDLE which informs
> RCU that the cpu is momentarily non-idle.
> 
> As an aside it is helpful to point out that the clock event device that is
> programmed here is not a per-cpu clock device; it is a
> pseudo clock device, used by the broadcast framework alone.
> The per-cpu clock device programming never goes through bc_set_next().
> 
> Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: linuxppc-dev@...abs.org
> Cc: mpe@...erman.id.au
> Cc: tglx@...utronix.de
> Link: http://lkml.kernel.org/r/20150318104705.17763.56668.stgit@preeti.in.ibm.com
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  kernel/time/tick-broadcast-hrtimer.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
> index eb682d5..6aac4be 100644
> --- a/kernel/time/tick-broadcast-hrtimer.c
> +++ b/kernel/time/tick-broadcast-hrtimer.c
> @@ -49,6 +49,7 @@ static void bc_set_mode(enum clock_event_mode mode,
>   */
>  static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
>  {
> +	int bc_moved;
>  	/*
>  	 * We try to cancel the timer first. If the callback is on
>  	 * flight on some other cpu then we let it handle it. If we
> @@ -60,9 +61,15 @@ static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
>  	 * restart the timer because we are in the callback, but we
>  	 * can set the expiry time and let the callback return
>  	 * HRTIMER_RESTART.
> +	 *
> +	 * Since we are in the idle loop at this point and because
> +	 * hrtimer_{start/cancel} functions call into tracing,
> +	 * calls to these functions must be bound within RCU_NONIDLE.
>  	 */
> -	if (hrtimer_try_to_cancel(&bctimer) >= 0) {
> -		hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED);
> +	RCU_NONIDLE(bc_moved = (hrtimer_try_to_cancel(&bctimer) >= 0) ?
> +		!hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED) :
> +			0);
> +	if (bc_moved) {
>  		/* Bind the "device" to the cpu */
>  		bc->bound_on = smp_processor_id();
>  	} else if (bc->bound_on == smp_processor_id()) {
> 

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