Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

[Kohli, Gaurav]

On 4/26/2018 2:27 PM, Peter Zijlstra wrote:

On Thu, Apr 26, 2018 at 10:41:31AM +0200, Peter Zijlstra wrote:

diff --git a/kernel/kthread.c b/kernel/kthread.c
index cd50e99202b0..4b6503c6a029 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -177,12 +177,13 @@ void *kthread_probe_data(struct task_struct *task)

static void __kthread_parkme(struct kthread *self)

  {
-       __set_current_state(TASK_PARKED);
-       while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
+       for (;;) {
+               __set_task_state(TASK_PARKED);

                set_current_state(TASK_PARKED);

of course..

Hi Peter,

Maybe i am missing something , but still that race can come as we don't put 
task_parked on special state.

Controller                                                                      
 Hotplug

                                                                                
 Loop

                                                                                
 Task_Interruptible

Set SHOULD_PARK

wakeup -> Proceeds

                                                                                
  Set Running

                                                                                
  kthread_parkme

                                                                                
  SET TASK_PARKED

                                                                                
  schedule

Set TASK_RUNNING

Can you please correct ME, if I misunderstood this.

+               if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
+                       break;
                if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
                        complete(&self->parked);
                schedule();
-               __set_current_state(TASK_PARKED);
        }
        clear_bit(KTHREAD_IS_PARKED, &self->flags);
        __set_current_state(TASK_RUNNING);

--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. 
is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.