Re: [PATCH] kernel/trace: Stop and wait for kthread on preempt irq module unload

[Xiao Yang]

On 2020/4/28 22:15, Joel Fernandes wrote:
I am wondering if it is because in your test, the kthread exits too quickly.
We have these comments in kthread_stop():
  * If threadfn() may call do_exit() itself, the caller must ensure
  * task_struct can't go away.
Does the below diff on top of the previous patch help?

---8<-----------------------

diff --git a/kernel/trace/preemptirq_delay_test.c 
b/kernel/trace/preemptirq_delay_test.c
index 1c28ca20e30b6..8051946a18989 100644
--- a/kernel/trace/preemptirq_delay_test.c
+++ b/kernel/trace/preemptirq_delay_test.c
@@ -152,6 +152,8 @@ static int __init preemptirq_delay_init(void)
        int retval;

        test_task = preemptirq_start_test();
+       get_task_struct(test_task);
+
        retval = PTR_ERR_OR_ZERO(test_task);
        if (retval != 0)
                return retval;
@@ -172,8 +174,10 @@ static void __exit preemptirq_delay_exit(void)
  {
        kobject_put(preemptirq_delay_kobj);

-       if (test_task)
+       if (test_task) {
                kthread_stop(test_task);
+               put_task_struct(test_task);
+       }
  }

Hi Joel,

Thanks for your additional patch.

First, We have to avoid kbuild error by including <linux/sched/task.h>
---------------------------------------
kernel/trace/preemptirq_delay_test.c: In function ‘preemptirq_delay_init’:
kernel/trace/preemptirq_delay_test.c:155:2: error: implicit declaration 
of function ‘get_task_struct’; did you mean ‘set_task_cpu’? 
[-Werror=implicit-function-declaration]
  get_task_struct(test_task);
  ^~~~~~~~~~~~~~~
  set_task_cpu
kernel/trace/preemptirq_delay_test.c: In function ‘preemptirq_delay_exit’:
kernel/trace/preemptirq_delay_test.c:179:3: error: implicit declaration 
of function ‘put_task_struct’; did you mean ‘set_task_cpu’? 
[-Werror=implicit-function-declaration]
   put_task_struct(test_task);
   ^~~~~~~~~~~~~~~
   set_task_cpu
cc1: some warnings being treated as errors
---------------------------------------

Second, I used the following steps to do test and didn't get any 
warning/panic after applying your additional patch：
---------------------------------------
for i in $(seq 1 100); do modprobe preemptirq_delay_test test_mode=irq 
delay=500000; rmmod preemptirq_delay_test; done
for i in $(seq 1 100); do modprobe preemptirq_delay_test 
test_mode=preempt delay=500000; rmmod preemptirq_delay_test; done
---------------------------------------

Thanks,
Xiao Yang