[PATCH] kernel/{lockdep,hung_task}: Show locks and backtrace of running tasks.

[Tetsuo Handa]

We are getting reports from syzbot where running task seems to be
relevant to a hung task problem but NMI backtrace does not print useful
information [1].

Although commit 8cc05c71ba5f7936 ("locking/lockdep: Move sanity check to
inside lockdep_print_held_locks()") says that calling
lockdep_print_held_locks() on a running thread is considered unsafe,
it is useful for syzbot to show locks and backtrace of running tasks.
Thus, let's allow it if CONFIG_DEBUG_AID_FOR_SYZBOT is defined.

[1] https://syzkaller.appspot.com/bug?id=8bab7a6a5597bb10f90e8227a7d8a483748d93be

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
---
 kernel/hung_task.c       | 20 ++++++++++++++++++++
 kernel/locking/lockdep.c |  9 +++++++++
 2 files changed, 29 insertions(+)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index b9132d1..1ac49a5 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -201,6 +201,26 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 	if (hung_task_show_lock)
 		debug_show_all_locks();
 	if (hung_task_call_panic) {
+#ifdef CONFIG_DEBUG_AID_FOR_SYZBOT
+		/*
+		 * debug_show_all_locks() above forcibly dumped locks held by
+		 * running tasks with locks held. Now, let's dump backtrace of
+		 * running tasks as well, for NMI backtrace below tends to show
+		 * current thread (i.e. khungtaskd thread itself) and idle CPU
+		 * which are useless for debugging hung task problems.
+		 */
+		rcu_read_lock();
+		for_each_process_thread(g, t) {
+			if (t->state != TASK_RUNNING || t == current)
+				continue;
+			pr_err("INFO: task %s:%d was running.\n", t->comm,
+			       t->pid);
+			sched_show_task(t);
+			touch_nmi_watchdog();
+			touch_all_softlockup_watchdogs();
+		}
+		rcu_read_unlock();
+#endif
 		trigger_all_cpu_backtrace();
 		panic("hung_task: blocked tasks");
 	}
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index e406c5f..efeebf6 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -565,12 +565,21 @@ static void lockdep_print_held_locks(struct task_struct *p)
 	else
 		printk("%d lock%s held by %s/%d:\n", depth,
 		       depth > 1 ? "s" : "", p->comm, task_pid_nr(p));
+#ifndef CONFIG_DEBUG_AID_FOR_SYZBOT
 	/*
 	 * It's not reliable to print a task's held locks if it's not sleeping
 	 * and it's not the current task.
 	 */
 	if (p->state == TASK_RUNNING && p != current)
 		return;
+#else
+	/*
+	 * But showing locks and backtrace of running tasks seems to be helpful
+	 * for debugging hung task problems. Since syzbot will call panic()
+	 * shortly, risking problems caused by accessing stale information is
+	 * acceptable here.
+	 */
+#endif
 	for (i = 0; i < depth; i++) {
 		printk(" #%d: ", i);
 		print_lock(p->held_locks + i);
-- 
1.8.3.1