Re: [PATCH] tracing: Wait for preempt irq delay thread to finish

[Xiao Yang]

Hi Steven,

Thanks for your further investigation.
I used the following ways to test your fix patch on my slow vm and 
didn't see any issue:
1) Insert and remove preemptirq_delay_test in loops.
2) Insert preemptirq_delay_test, write to 
/sys/kernel/preemptirq_delay_test/trigger and remove 
preemptirq_delay_test in loops.
3) Ran irqsoff_tracer.tc in loops.

BTW: For irqsoff_tracer.tc, should we extend code to test the burst 
feature and the sysfs trigger?

Reviewed-by: Xiao Yang <yangx...@cn.fujitsu.com>

Thanks,
Xiao Yang
On 2020/5/6 22:30, Steven Rostedt wrote:
From: "Steven Rostedt (VMware)"<rost...@goodmis.org>

Running on a slower machine, it is possible that the preempt delay kernel
thread may still be executing if the module was immediately removed after
added, and this can cause the kernel to crash as the kernel thread might be
executing after its code has been removed.

There's no reason that the caller of the code shouldn't just wait for the
delay thread to finish, as the thread can also be created by a trigger in
the sysfs code, which also has the same issues.

Link: http://lore.kernel.org/r/5ea2b0c8.2080...@cn.fujitsu.com

Cc: sta...@vger.kernel.org
Fixes: 793937236d1ee ("lib: Add module for testing preemptoff/irqsoff latency 
tracers")
Reported-by: Xiao Yang<yangx...@cn.fujitsu.com>
Signed-off-by: Steven Rostedt (VMware)<rost...@goodmis.org>
---
  kernel/trace/preemptirq_delay_test.c | 30 ++++++++++++++++++++++------
  1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/preemptirq_delay_test.c 
b/kernel/trace/preemptirq_delay_test.c
index 31c0fad4cb9e..c4c86de63cf9 100644
--- a/kernel/trace/preemptirq_delay_test.c
+++ b/kernel/trace/preemptirq_delay_test.c
@@ -113,22 +113,42 @@ static int preemptirq_delay_run(void *data)

        for (i = 0; i<  s; i++)
                (testfuncs[i])(i);
+
+       set_current_state(TASK_INTERRUPTIBLE);
+       while (!kthread_should_stop()) {
+               schedule();
+               set_current_state(TASK_INTERRUPTIBLE);
+       }
+
+       __set_current_state(TASK_RUNNING);
+
        return 0;
  }

-static struct task_struct *preemptirq_start_test(void)
+static int preemptirq_run_test(void)
  {
+       struct task_struct *task;
+
        char task_name[50];

        snprintf(task_name, sizeof(task_name), "%s_test", test_mode);
-       return kthread_run(preemptirq_delay_run, NULL, task_name);
+       task =  kthread_run(preemptirq_delay_run, NULL, task_name);
+       if (IS_ERR(task))
+               return PTR_ERR(task);
+       if (task)
+               kthread_stop(task);
+       return 0;
  }


  static ssize_t trigger_store(struct kobject *kobj, struct kobj_attribute 
*attr,
                         const char *buf, size_t count)
  {
-       preemptirq_start_test();
+       ssize_t ret;
+
+       ret = preemptirq_run_test();
+       if (ret)
+               return ret;
        return count;
  }

@@ -148,11 +168,9 @@ static struct kobject *preemptirq_delay_kobj;

  static int __init preemptirq_delay_init(void)
  {
-       struct task_struct *test_task;
        int retval;

-       test_task = preemptirq_start_test();
-       retval = PTR_ERR_OR_ZERO(test_task);
+       retval = preemptirq_run_test();
        if (retval != 0)
                return retval;