linux-kernel - [PATCH] timers/core: Correct callback order during CPU hot plug.

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Date:	Wed, 27 Jul 2016 11:08:18 +0200
From:	Richard Cochran <rcochran@...utronix.de>
To:	<linux-kernel@...r.kernel.org>
Cc:	<rt@...utronix.de>, John Stultz <john.stultz@...aro.org>,
	Jon Hunter <jonathanh@...dia.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] timers/core: Correct callback order during CPU hot plug.

On the tear-down path, the dead CPU callback for the timers was
misplaced within the 'cpuhp_state' enumeration.  There is a hidden
dependency between the timers and block multiqueue.  The timers
callback must happen before the block multiqueue callback otherwise a
RCU stall occurs.

This patch fixes the order within the hot plug state machine.

Signed-off-by: Richard Cochran <rcochran@...utronix.de>
Fixes: 24f73b99716a ("timers/core: Convert to hotplug state machine")

Cc: John Stultz <john.stultz@...aro.org>
Cc: Jon Hunter <jonathanh@...dia.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: rt@...utronix.de
---
 include/linux/cpuhotplug.h |  2 +-
 kernel/cpu.c               | 15 ++++++++++-----
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 6d405db..242bf53 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -20,9 +20,9 @@ enum cpuhp_state {
 	CPUHP_PROFILE_PREPARE,
 	CPUHP_X2APIC_PREPARE,
 	CPUHP_SMPCFD_PREPARE,
-	CPUHP_TIMERS_DEAD,
 	CPUHP_RCUTREE_PREP,
 	CPUHP_NOTIFY_PREPARE,
+	CPUHP_TIMERS_DEAD,
 	CPUHP_BRINGUP_CPU,
 	CPUHP_AP_IDLE_DEAD,
 	CPUHP_AP_OFFLINE,
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 67f4943..ec12b72 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1208,11 +1208,6 @@ static struct cpuhp_step cpuhp_bp_states[] = {
 		.startup = smpcfd_prepare_cpu,
 		.teardown = smpcfd_dead_cpu,
 	},
-	[CPUHP_TIMERS_DEAD] = {
-		.name = "timers dead",
-		.startup = NULL,
-		.teardown = timers_dead_cpu,
-	},
 	[CPUHP_RCUTREE_PREP] = {
 		.name = "RCU-tree prepare",
 		.startup = rcutree_prepare_cpu,
@@ -1229,6 +1224,16 @@ static struct cpuhp_step cpuhp_bp_states[] = {
 		.skip_onerr		= true,
 		.cant_stop		= true,
 	},
+	/*
+	 * On the tear-down path, timers_dead_cpu() must be invoked
+	 * before blk_mq_queue_reinit_notify() from notify_dead(),
+	 * otherwise a RCU stall occurs.
+	 */
+	[CPUHP_TIMERS_DEAD] = {
+		.name = "timers dead",
+		.startup = NULL,
+		.teardown = timers_dead_cpu,
+	},
 	/* Kicks the plugged cpu into life */
 	[CPUHP_BRINGUP_CPU] = {
 		.name			= "cpu:bringup",
-- 
2.1.4