Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

[Kohli, Gaurav]

Hi ,

We can also fix below race by smpboot code as well:

@@ -109,7 +109,6 @@ static int smpboot_thread_fn(void *data)

        struct smp_hotplug_thread *ht = td->ht;

        while (1) {

-               set_current_state(TASK_INTERRUPTIBLE);

                preempt_disable();

                if (kthread_should_stop()) {

                        __set_current_state(TASK_RUNNING);

@@ -157,6 +156,7 @@ static int smpboot_thread_fn(void *data)

                if (!ht->thread_should_run(td->cpu)) {

                        preempt_enable_no_resched();

+                       set_current_state(TASK_INTERRUPTIBLE);

                        schedule();

                } else {

                        __set_current_state(TASK_RUNNING);

Please suggest if this approach is better.

Regards

Gaurav

On 4/24/2018 2:58 PM, Gaurav Kohli wrote:

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.

Suggested-by: Pavankumar Kondeti <pkond...@codeaurora.org>
Co-developed-by: Neeraj Upadhyay <neer...@codeaurora.org>
Signed-off-by: Neeraj Upadhyay <neer...@codeaurora.org>
Signed-off-by: Gaurav Kohli <gko...@codeaurora.org>

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 1650578..514b232 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data)
                }

if (kthread_should_park()) {

+                       raw_spin_lock(&current->pi_lock);
                        __set_current_state(TASK_RUNNING);
+                       raw_spin_unlock(&current->pi_lock);
                        preempt_enable();
                        if (ht->park && td->status == HP_THREAD_ACTIVE) {
                                BUG_ON(td->cpu != smp_processor_id());

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