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

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Message-ID: <tip-a127d2bcf1fbc8c8e0b5cf0dab54f7d3ff50ce47@git.kernel.org>
Date:	Mon, 23 Mar 2015 05:24:54 -0700
From:	tip-bot for Preeti U Murthy <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, tglx@...utronix.de,
	peterz@...radead.org, paulmck@...ux.vnet.ibm.com,
	preeti@...ux.vnet.ibm.com, hpa@...or.com
Subject: [tip:timers/urgent] timers/tick/broadcast-hrtimer:
  Fix suspicious RCU usage in idle loop

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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