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Subject[PATCH] timers/core: Correct callback order during CPU hot plug.
Date
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@linutronix.de>
Fixes: 24f73b99716a ("timers/core: Convert to hotplug state machine")

Cc: John Stultz <john.stultz@linaro.org>
Cc: Jon Hunter <jonathanh@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: rt@linutronix.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
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