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Subject[tip: sched/core] cpu/hotplug, stop_machine: Fix stop_machine vs hotplug order
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 45178ac0cea853fe0e405bf11e101bdebea57b15
Gitweb: https://git.kernel.org/tip/45178ac0cea853fe0e405bf11e101bdebea57b15
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 10 Dec 2019 09:34:54 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 17 Dec 2019 13:32:50 +01:00

cpu/hotplug, stop_machine: Fix stop_machine vs hotplug order

Paul reported a very sporadic, rcutorture induced, workqueue failure.
When the planets align, the workqueue rescuer's self-migrate fails and
then triggers a WARN for running a work on the wrong CPU.

Tejun then figured that set_cpus_allowed_ptr()'s stop_one_cpu() call
could be ignored! When stopper->enabled is false, stop_machine will
insta complete the work, without actually doing the work. Worse, it
will not WARN about this (we really should fix this).

It turns out there is a small window where a freshly online'ed CPU is
marked 'online' but doesn't yet have the stopper task running:

BP AP

bringup_cpu()
__cpu_up(cpu, idle) --> start_secondary()
...
cpu_startup_entry()
bringup_wait_for_ap()
wait_for_ap_thread() <-- cpuhp_online_idle()
while (1)
do_idle()

... available to run kthreads ...

stop_machine_unpark()
stopper->enable = true;

Close this by moving the stop_machine_unpark() into
cpuhp_online_idle(), such that the stopper thread is ready before we
start the idle loop and schedule.

Reported-by: "Paul E. McKenney" <paulmck@kernel.org>
Debugged-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: "Paul E. McKenney" <paulmck@kernel.org>
---
kernel/cpu.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index a59cc98..e7f7967 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -525,8 +525,7 @@ static int bringup_wait_for_ap(unsigned int cpu)
if (WARN_ON_ONCE((!cpu_online(cpu))))
return -ECANCELED;

- /* Unpark the stopper thread and the hotplug thread of the target cpu */
- stop_machine_unpark(cpu);
+ /* Unpark the hotplug thread of the target cpu */
kthread_unpark(st->thread);

/*
@@ -1089,8 +1088,8 @@ void notify_cpu_starting(unsigned int cpu)

/*
* Called from the idle task. Wake up the controlling task which brings the
- * stopper and the hotplug thread of the upcoming CPU up and then delegates
- * the rest of the online bringup to the hotplug thread.
+ * hotplug thread of the upcoming CPU up and then delegates the rest of the
+ * online bringup to the hotplug thread.
*/
void cpuhp_online_idle(enum cpuhp_state state)
{
@@ -1100,6 +1099,12 @@ void cpuhp_online_idle(enum cpuhp_state state)
if (state != CPUHP_AP_ONLINE_IDLE)
return;

+ /*
+ * Unpart the stopper thread before we start the idle loop (and start
+ * scheduling); this ensures the stopper task is always available.
+ */
+ stop_machine_unpark(smp_processor_id());
+
st->state = CPUHP_AP_ONLINE_IDLE;
complete_ap_thread(st, true);
}
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