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sched: Queue a deboosted task to the head of the RT prio 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> Acked-by: Peter Zijlstra <peterz@infradead.org> Tested-by: Carsten Emde <cbe@osadl.org> Tested-by: Mathias Weber <mathias.weber.mw1@roche.com> LKML-Reference: <20100120171629.809074113@linutronix.de>
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@ -4237,7 +4237,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
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if (running)
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p->sched_class->set_curr_task(rq);
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if (on_rq) {
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enqueue_task(rq, p, 0, false);
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enqueue_task(rq, p, 0, oldprio < prio);
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check_class_changed(rq, p, prev_class, oldprio, running);
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}
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