linux-kernel - [PATCH] wchan: Fix get_wchan() when task in schedule

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Message-ID: <20230330121238.176534-1-chenzhongjin@huawei.com>
Date:   Thu, 30 Mar 2023 20:12:37 +0800
From:   Chen Zhongjin <chenzhongjin@...wei.com>
To:     <linux-kernel@...r.kernel.org>
CC:     <chenzhongjin@...wei.com>, <mingo@...hat.com>,
        <peterz@...radead.org>, <juri.lelli@...hat.com>,
        <vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
        <rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
        <bristot@...hat.com>, <vschneid@...hat.com>,
        <rmk+kernel@...linux.org.uk>, <geert@...ux-m68k.org>,
        <keescook@...omium.org>
Subject: [PATCH] wchan: Fix get_wchan() when task in schedule

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 can prevent
this.

Fixes: 42a20f86dc19 ("sched: Add wrapper for get_wchan() to keep task blocked")
Signed-off-by: Chen Zhongjin <chenzhongjin@...wei.com>
---
 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d18c3969f90..2071d1c0eaca 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2041,7 +2041,8 @@ 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)
+	if (state != TASK_RUNNING && state != TASK_WAKING &&
+	    !p->on_rq && !p->on_cpu)
 		ip = __get_wchan(p);
 	raw_spin_unlock_irq(&p->pi_lock);
 
-- 
2.17.1