1 files changed, 19 insertions, 1 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 1803667dbb6e..32d7eb474fa7 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6714,7 +6714,25 @@ recheck:
         if (running)
                 p->sched_class->set_curr_task(rq);
         if (on_rq) {
-                activate_task(rq, p, 0, false);
+                /*
+                 * Workaround to make prio ceiling work as expected:
+                 *
+                 * Queue task to head when task is running and task is
+                 * lowering its priority. This works around the non-
+                 * availability of a sched_setprio syscall which was
+                 * tinkered into the posix spec to make prio ceiling
+                 * work correctly.
+                 *
+                 * This workaround violates the posix scheduling
+                 * semantics of tail queueing in the case that the
+                 * priority was changed by anything else than
+                 * sched_setprio, but there is no other breakage
+                 * lurking than some specification fetishists going
+                 * berserk on me.
+                 *
+                 * Fixing this in mainline needs more thoughts.
+                 */
+                activate_task(rq, p, 0, running && oldprio < p->prio);
 
                 check_class_changed(rq, p, prev_class, oldprio, running);
         }