[PATCH v3] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

["Luis Claudio R. Goncalves" <lgoncalv@redhat.com>]

With PREEMPT_RT enabled, some of the calls to put_task_struct() coming
from rt_mutex_adjust_prio_chain() could happen in preemptible context and
with a mutex enqueued. That could lead to this sequence:

	rt_mutex_adjust_prio_chain()
	  put_task_struct()
	    __put_task_struct()
	      sched_ext_free()
	        spin_lock_irqsave()
	          rtlock_lock() --->  TRIGGERS
	                              lockdep_assert(!current->pi_blocked_on);

Fix that by unconditionally resorting to the deferred call to
__put_task_struct().

v2: (Rostedt) remove the #ifdef from put_task_struct() and create
    tsk_is_pi_blocked_on() in sched.h to make the change cleaner.

v3: (Sebastian and PeterZ) always call the RCU deferred __put_task_struct().

Suggested-by: Crystal Wood <crwood@redhat.com>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
---
 include/linux/sched/task.h |   20 +++++---------------
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 0f2aeb37bbb04..49847efe5559e 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -134,22 +134,12 @@ static inline void put_task_struct(struct task_struct *t)
 		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 !RT, it is always safe to call __put_task_struct(),
+	 * but under PREEMPT_RT, we can't call put_task_struct
 	 * in atomic context because it will indirectly
-	 * acquire sleeping locks.
+	 * acquire sleeping locks. The same is true if the
+	 * current process has a mutex enqueued (blocked on
+	 * a PI chain).
 	 *
 	 * call_rcu() will schedule delayed_put_task_struct_rcu()
 	 * to be called in process context.
-- 
2.49.0