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

From: tip-bot for Thomas Gleixner
Date: Fri Jan 22 2010 - 12:14:03 EST

Commit-ID: 60db48cacb9b253d5607a5ff206112a59cd09e34
Gitweb: http://git.kernel.org/tip/60db48cacb9b253d5607a5ff206112a59cd09e34
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Wed, 20 Jan 2010 20:59:06 +0000
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 22 Jan 2010 18:09:59 +0100

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@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>
kernel/sched.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index f47560f..a56ead4 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4237,7 +4237,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
if (running)
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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