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.

Move the timers callback to the proper place in the state machine.

Reported-and-tested-by: Jon Hunter <jonathanh@nvidia.com>
Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 24f73b99716a ("timers/core: Convert to hotplug state machine")
Signed-off-by: Richard Cochran <rcochran@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: John Stultz <john.stultz@linaro.org>
Cc: rt@linutronix.de
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1469610498-25914-1-git-send-email-rcochran@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 11 insertions, 6 deletions

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 6d405db6fb12..242bf530edfc 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 f24f45915b54..341bf80f80bd 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1200,11 +1200,6 @@ 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,
@@ -1221,6 +1216,16 @@ 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",