[patch 3/3] sched: Queue a deboosted task to the head of the RT priority queue

[Thomas Gleixner <tglx@linutronix.de>]

[-- Attachment #1: rtmutex-set-prio-fix-deboost-enqueueing.patch --]
[-- Type: text/plain, Size: 1436 bytes --]

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>
---
 kernel/sched.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6-tip/kernel/sched.c
===================================================================
--- linux-2.6-tip.orig/kernel/sched.c
+++ linux-2.6-tip/kernel/sched.c
@@ -6066,7 +6066,7 @@ void rt_mutex_setprio(struct task_struct
 	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);
 	}