From 363da406f01999f1e8e035f57b15d01368dcef94 Mon Sep 17 00:00:00 2001
From: Wander Lairson Costa <wander@redhat.com>
Date: Wed, 14 Jun 2023 09:23:22 -0300
Subject: [PATCH 174/184] sched: avoid false lockdep splat in put_task_struct()

In put_task_struct(), a spin_lock is indirectly acquired under the kernel
stock. When running the kernel in real-time (RT) configuration, the
operation is dispatched to a preemptible context call to ensure
guaranteed preemption. However, if PROVE_RAW_LOCK_NESTING is enabled
and __put_task_struct() is called while holding a raw_spinlock, lockdep
incorrectly reports an "Invalid lock context" in the stock kernel.

This false splat occurs because lockdep is unaware of the different
route taken under RT. To address this issue, override the inner wait
type to prevent the false lockdep splat.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230614122323.37957-3-wander@redhat.com
(cherry picked from commit 893cdaaa3977be6afb3a7f756fbfd7be83f68d8c)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
---
 include/linux/sched/task.h | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index ff5aaed609695..d4861ba862167 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -116,6 +116,19 @@ static inline void put_task_struct(struct task_struct *t)
 	if (!refcount_dec_and_test(&t->usage))
 		return;
 
+	/*
+	 * In !RT, it is always safe to call __put_task_struct().
+	 * Under RT, we can only call it in preemptible context.
+	 */
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
+		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
+
+		lock_map_acquire_try(&put_task_map);
+		__put_task_struct(t);
+		lock_map_release(&put_task_map);
+		return;
+	}
+
 	/*
 	 * under PREEMPT_RT, we can't call put_task_struct
 	 * in atomic context because it will indirectly
@@ -136,10 +149,7 @@ static inline void put_task_struct(struct task_struct *t)
 	 * when it fails to fork a process. Therefore, there is no
 	 * way it can conflict with put_task_struct().
 	 */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
-		call_rcu(&t->rcu, __put_task_struct_rcu_cb);
-	else
-		__put_task_struct(t);
+	call_rcu(&t->rcu, __put_task_struct_rcu_cb);
 }
 
 DEFINE_FREE(put_task, struct task_struct *, if (_T) put_task_struct(_T))
-- 
2.55.0

