From a5e204282a8c65f3d810ba03414c436a292b695d Mon Sep 17 00:00:00 2001 From: Gaurav Kohli Date: Tue, 24 Apr 2018 12:13:34 +0530 Subject: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup The control cpu thread which initiates hotplug calls kthread_park() for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control thread wakes up the hotplug thread. There is a chance that wakeup code sees the hotplug thread (running on AP core) in INTERRUPTIBLE state, but sets its state to RUNNING after hotplug thread has entered kthread_parkme() and changed its state to TASK_PARKED. This can result in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED flag set but fails to rebind the kthread, due to it being not in TASK_PARKED state. Fix this, by serializing wakeup state change, against state change before parking the kthread. Below is the possible race: Control thread Hotplug Thread kthread_park() set KTHREAD_SHOULD_PARK smpboot_thread_fn set_current_state(TASK_INTERRUPTIBLE); kthread_parkme wake_up_process() raw_spin_lock_irqsave(&p->pi_lock, flags); if (!(p->state & state)) -> this will fail goto out; __kthread_parkme __set_current_state(TASK_PARKED); if (p->on_rq && ttwu_remote(p, wake_flags)) ttwu_remote() p->state = TASK_RUNNING; schedule(); So to avoid this race, take pi_lock to serial state changes. Change-Id: Ie71645d37046f7ee74df880dbead29efbaad199a Suggested-by: Pavankumar Kondeti Signed-off-by: Neeraj Upadhyay Signed-off-by: Gaurav Kohli --- kernel/smpboot.c | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) diff --git a/kernel/smpboot.c b/kernel/smpboot.c index 3a0415803b09..552e154fc77e 100644 --- a/kernel/smpboot.c +++ b/kernel/smpboot.c @@ -122,7 +122,45 @@ static int smpboot_thread_fn(void *data) } if (kthread_should_park()) { + /* + * Serialize against wakeup. If we take the lock first, + * wakeup is skipped. If we run later, we observe, + * TASK_RUNNING update from wakeup path, before moving + * forward. This helps avoid the race, where wakeup + * observes TASK_INTERRUPTIBLE, and also observes + * the TASK_PARKED in kthread_parkme() before updating + * task state to TASK_RUNNING. In this case, kthread + * gets parked in TASK_RUNNING state. This results + * in panic later on in kthread_unpark(), as it sees + * KTHREAD_IS_PARKED flag set but fails to rebind the + * kthread, due to it being not in TASK_PARKED state. + * + * Control thread Hotplug Thread + * + * kthread_park() + * set KTHREAD_SHOULD_PARK + * smpboot_thread_fn() + * set_current_state( + * TASK_INTERRUPTIBLE); + * kthread_parkme() + * + * wake_up_process() + * + * raw_spin_lock_irqsave(&p->pi_lock, flags); + * if (!(p->state & state)) + * goto out; + * + * __set_current_state( + * TASK_PARKED); + * + * if (p->on_rq && ttwu_remote(p, wake_flags)) + * ttwu_remote() + * p->state = TASK_RUNNING; + * schedule(); + */ + raw_spin_lock(¤t->pi_lock); __set_current_state(TASK_RUNNING); + raw_spin_unlock(¤t->pi_lock); preempt_enable(); if (ht->park && td->status == HP_THREAD_ACTIVE) { BUG_ON(td->cpu != smp_processor_id()); -- 2.11.0