[091/123] sched: Queue a deboosted task to the head of the RT prio queue

[Greg KH]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit 60db48cacb9b253d5607a5ff206112a59cd09e34 upstream

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@xxxxxxxxx>
Reported-by: Carsten Emde <cbe@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Tested-by: Carsten Emde <cbe@xxxxxxxxx>
Tested-by: Mathias Weber <mathias.weber.mw1@xxxxxxxxx>
LKML-Reference: <20100120171629.809074113@xxxxxxxxxxxxx>
Signed-off-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
---
 kernel/sched.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6184,7 +6184,7 @@ 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);
 	}


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