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

[Kohli, Gaurav]

On 4/24/2018 11:56 PM, Peter Zijlstra wrote:

On Tue, Apr 24, 2018 at 02:58:25PM +0530, 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());

Note how in your scenario above you didn't actually need the
TASK_RUNNING state; so how is this change going to fix anything?

Hi Peter,

As with help of this , if kthread_should_park run first so wake_up call of 
controller

get exited as task is already set as running, otherwise if controller runs first

then we will block here and set running and then sets TASK_PARKED .

So no chance of cpuhp set as running during  kthread_parkme call.

But as we discussed this can be fix by 2nd patch as well, So once you get time 
and able to

see , Please let us know or do you want me to try your 2nd patch

for testing first?

But yes, I suspect it is right, but it definitely needs a comment
explaining wth we take that lock there.

Like I said earlier, my brain is entirely fried for the day; but I'll
have a try tomorrow.

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