[PATCH] sched: Handle priority boosted tasks proper in setscheduler()

[Thomas Gleixner <tglx@linutronix.de>]

Ronny reported that the following scenario is not handled correctly:

T1 (prio = 10)
   lock(rtmutex);

T2 (prio = 20)
   lock(rtmutex)
      boost T1

T1 (prio = 20)
   sys_set_scheduler(prio = 30)
   T1 prio = 30
   ....
   sys_set_scheduler(prio = 10)
   T1 prio = 30

The last step is wrong as T1 should now be back at prio 20.

commit c365c292d0590 "sched: Consider pi boosting in setscheduler()"
only handles the case where a boosted tasks tries to lower its
priority.

Fix it by taking the new effective priority into account for the
decision whether a change of the priority is required.

Reported-by: Ronny Meeus <ronny.meeus@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/locking/rtmutex.c |   10 ++++++----
 kernel/sched/core.c      |   11 +++++------
 2 files changed, 11 insertions(+), 10 deletions(-)

Index: tip/kernel/locking/rtmutex.c
===================================================================
--- tip.orig/kernel/locking/rtmutex.c
+++ tip/kernel/locking/rtmutex.c
@@ -265,15 +265,17 @@ struct task_struct *rt_mutex_get_top_tas
 }
 
 /*
- * Called by sched_setscheduler() to check whether the priority change
- * is overruled by a possible priority boosting.
+ * Called by sched_setscheduler() to get the priority which will be
+ * effective after the change.
  */
 int rt_mutex_check_prio(struct task_struct *task, int newprio)
 {
 	if (!task_has_pi_waiters(task))
-		return 0;
+		return newprio;
 
-	return task_top_pi_waiter(task)->task->prio <= newprio;
+	if (task_top_pi_waiter(task)->task->prio <= newprio)
+		return task_top_pi_waiter(task)->task->prio;
+	return newprio;
 }
 
 /*
Index: tip/kernel/sched/core.c
===================================================================
--- tip.orig/kernel/sched/core.c
+++ tip/kernel/sched/core.c
@@ -3414,7 +3414,7 @@ static int __sched_setscheduler(struct t
 	int newprio = dl_policy(attr->sched_policy) ? MAX_DL_PRIO - 1 :
 		      MAX_RT_PRIO - 1 - attr->sched_priority;
 	int retval, oldprio, oldpolicy = -1, queued, running;
-	int policy = attr->sched_policy;
+	int new_effective_prio, policy = attr->sched_policy;
 	unsigned long flags;
 	const struct sched_class *prev_class;
 	struct rq *rq;
@@ -3596,15 +3596,14 @@ change:
 	oldprio = p->prio;
 
 	/*
-	 * Special case for priority boosted tasks.
-	 *
-	 * If the new priority is lower or equal (user space view)
-	 * than the current (boosted) priority, we just store the new
+	 * Take priority boosted tasks into account. If the new
+	 * effective priority is unchanged, we just store the new
 	 * normal parameters and do not touch the scheduler class and
 	 * the runqueue. This will be done when the task deboost
 	 * itself.
 	 */
-	if (rt_mutex_check_prio(p, newprio)) {
+	new_effective_prio = rt_mutex_check_prio(p, newprio);
+	if (new_effective_prio == oldprio) {
 		__setscheduler_params(p, attr);
 		task_rq_unlock(rq, p, &flags);
 		return 0;