[tip: sched/core] sched: Do not call __put_task_struct() on rt if pi_blocked_on is set

["tip-bot2 for Luis Claudio R. Goncalves" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     8671bad873ebeb082afcf7b4501395c374da6023
Gitweb:        https://git.kernel.org/tip/8671bad873ebeb082afcf7b4501395c374da6023
Author:        Luis Claudio R. Goncalves <lgoncalv@redhat.com>
AuthorDate:    Mon, 07 Jul 2025 11:03:59 -03:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 14 Jul 2025 17:16:33 +02:00

sched: Do not call __put_task_struct() on rt if pi_blocked_on is set

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);

This is not a SCHED_EXT bug. The first cleanup function called by
__put_task_struct() is sched_ext_free() and it happens to take a
(RT) spin_lock, which in the scenario described above, would trigger
the lockdep assertion of "!current->pi_blocked_on".

Crystal Wood was able to identify the problem as __put_task_struct()
being called during rt_mutex_adjust_prio_chain(), in the context of
a process with a mutex enqueued.

Instead of adding more complex conditions to decide when to directly
call __put_task_struct() and when to defer the call, unconditionally
resort to the deferred call on PREEMPT_RT to simplify the code.

Fixes: 893cdaaa3977 ("sched: avoid false lockdep splat in put_task_struct()")
Suggested-by: Crystal Wood <crwood@redhat.com>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/aGvTz5VaPFyj0pBV@uudg.org
---
 include/linux/sched/task.h | 27 ++++++++++-----------------
 1 file changed, 10 insertions(+), 17 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index c517dbc..ea41795 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -131,24 +131,17 @@ 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
+	 * 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).
+	 *
+	 * In !RT, it is always safe to call __put_task_struct().
+	 * Though, in order to simplify the code, resort to the
+	 * deferred call too.
 	 *
-	 * call_rcu() will schedule delayed_put_task_struct_rcu()
+	 * call_rcu() will schedule __put_task_struct_rcu_cb()
 	 * to be called in process context.
 	 *
 	 * __put_task_struct() is called when
@@ -161,7 +154,7 @@ static inline void put_task_struct(struct task_struct *t)
 	 *
 	 * delayed_free_task() also uses ->rcu, but it is only called
 	 * when it fails to fork a process. Therefore, there is no
-	 * way it can conflict with put_task_struct().
+	 * way it can conflict with __put_task_struct().
 	 */
 	call_rcu(&t->rcu, __put_task_struct_rcu_cb);
 }