sched: Queue a deboosted task to the head of the RT priority queue

rtmutex_set_prio() is used to implement priority inheritance for
futexes. When a task is deboosted it gets enqueued at the tail of its RT
priority list. This is violating the POSIX scheduling semantics:

rt priority list X contains two runnable tasks A and B

    task A	 runs with priority X and holds mutex M
    task C	 preempts A and is blocked on mutex M
         	 -> task A is boosted to priority of task C (Y)
    task A	 unlocks the mutex M and deboosts itself
         	 -> A is dequeued from rt priority list Y
    	 -> A is enqueued to the tail of rt priority list X
    task C	 schedules away
    task B	 runs

This is wrong as task A did not schedule away and therefor violates
the POSIX scheduling semantics.

Enqueue the task to the head of the priority list instead.

Reported-by: Mathias Weber <mathias.weber.mw1@roche.com>
Reported-by: Carsten Emde <cbe@osadl.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 8da004bbc8db..8262cf2de797 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6390,7 +6390,7 @@ void task_setprio(struct task_struct *p, int prio)
 	if (running)
 		p->sched_class->set_curr_task(rq);
 	if (on_rq) {
-		enqueue_task(rq, p, 0, false);
+		enqueue_task(rq, p, 0, oldprio < prio);
 
 		check_class_changed(rq, p, prev_class, oldprio, running);
 	}