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

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Message-ID: <20150226032202.20019.91636.stgit@preeti.in.ibm.com>
Date:	Thu, 26 Feb 2015 08:52:02 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	peterz@...radead.org, tglx@...utronix.de
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] 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. 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.

Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---

 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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