[RESEND PATCH v4] 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() if PREEMPT_RT is enabled.

Suggested-by: Crystal Wood <crwood@redhat.com>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
---

Resent as a gentle reminder, because this issue results in scary backtraces,
not obvious to debug and pinpoint root cause.

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, PeterZ) always call the deferred __put_task_struct() on RT.
v4: Fix the implementation of what was requested on v3.

 include/linux/sched/task.h |   17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 0f2aeb37bbb04..51678a541477a 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -134,11 +134,8 @@ 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()) {
+	/* In !RT, it is always safe to call __put_task_struct(). */
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
 		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
 
 		lock_map_acquire_try(&put_task_map);
@@ -148,11 +145,13 @@ static inline void put_task_struct(struct task_struct *t)
 	}
 
 	/*
-	 * 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).
 	 *
-	 * 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
@@ -165,7 +164,7 @@ 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);
 }

----- End forwarded message -----