Commit 8671bad8 authored Jul 07, 2025 by Luis Claudio R. Goncalves Committed by Peter Zijlstra Jul 14, 2025

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: 893cdaaa ("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

parent 7de9d4f9

include/linux/sched/task.h

+10 −17

Original line number	Diff line number	Diff line
		@@ -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);
		}