[PATCH v10 2/2] sched: avoid false lockdep splat in put_task_struct()

[Wander Lairson Costa <wander@redhat.com>]

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.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Luis Goncalves <lgoncalv@redhat.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 d20de91e3b95..b53909027771 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -125,6 +125,19 @@ 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
@@ -145,10 +158,7 @@ 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);
 }
 
 static inline void put_task_struct_many(struct task_struct *t, int nr)
-- 
2.40.1