[PATCH v2] wchan: Fix get_wchan() when task in schedule

[Chen Zhongjin]

get_wchan() check task to unwind is not running or going to run by:
state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq

However this cannot detect task which is going to be scheduled out.
For example, in this path:

  __wait_for_common(x, schedule_timeout, timeout, TASK_UNINTERRUPTIBLE)
  do_wait_for_common() // state == TASK_UNINTERRUPTIBLE
  schedule_timeout()
  __schedule()
    deactivate_task() // on_rq = 0

After this point get_wchan() can be run on the task but it is still
running actually, and p->pi_lock doesn't work for this case.

It can trigger some warning when running stacktrace on a running task.
Also check p->on_cpu to promise task is really switched out to prevent
this.

Fixes: 42a20f86dc19 ("sched: Add wrapper for get_wchan() to keep task blocked")
Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
---
v1 -> v2:
Fix task_struct->on_cpu not exist for !CONFIG_SMP
---
 kernel/sched/core.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d18c3969f90..94799db69487 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2033,6 +2033,7 @@ unsigned long get_wchan(struct task_struct *p)
 {
 	unsigned long ip = 0;
 	unsigned int state;
+	int on_cpu = 0;
 
 	if (!p || p == current)
 		return 0;
@@ -2041,7 +2042,12 @@ unsigned long get_wchan(struct task_struct *p)
 	raw_spin_lock_irq(&p->pi_lock);
 	state = READ_ONCE(p->__state);
 	smp_rmb(); /* see try_to_wake_up() */
-	if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq)
+
+#ifdef CONFIG_SMP
+	on_cpu = p->on_cpu;
+#endif
+	if (state != TASK_RUNNING && state != TASK_WAKING &&
+	    !p->on_rq && !on_cpu)
 		ip = __get_wchan(p);
 	raw_spin_unlock_irq(&p->pi_lock);
 
-- 
2.17.1