On Fri, 2012-10-26 at 10:03 -0700, Mike Galbraith wrote:

The bug is in the patch that used sched_setscheduler_nocheck().  Plain
sched_setscheduler() would have replied -EGOAWAY.

sched_setscheduler_nocheck() should say go away too methinks.  This
isn't about permissions, it's about not being stupid in general.

sched: fix __sched_setscheduler() RT_GROUP_SCHED conditionals

Remove user and rt_bandwidth_enabled() RT_GROUP_SCHED conditionals in
__sched_setscheduler().  The end result of kernel OR user promoting a
task in a group with zero rt_runtime allocated is the same bad thing,
and throttle switch position matters little.  It's safer to just say
no solely based upon bandwidth existence, may save the user a nasty
surprise if he later flips the throttle switch to 'on'.

The commit below came about due to sched_setscheduler_nocheck()
allowing a task in a task group with zero rt_runtime allocated to
be promoted by the kernel oom logic, thus marooning it forever.

<quote>
commit 341aea2bc48bf652777fb015cc2b3dfa9a451817
Author: KOSAKI Motohiro [off-list ref]
Date:   Thu Apr 14 15:22:13 2011 -0700

    oom-kill: remove boost_dying_task_prio()
    
    This is an almost-revert of commit 93b43fa ("oom: give the dying task a
    higher priority").
    
    That commit dramatically improved oom killer logic when a fork-bomb
    occurs.  But I've found that it has nasty corner case.  Now cpu cgroup has
    strange default RT runtime.  It's 0!  That said, if a process under cpu
    cgroup promote RT scheduling class, the process never run at all.
</quote>

Signed-off-by: Mike Galbraith <redacted>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d8927f..d3a35f8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -3810,17 +3810,14 @@ recheck:
 	}
 
 #ifdef CONFIG_RT_GROUP_SCHED
-	if (user) {
-		/*
-		 * Do not allow realtime tasks into groups that have no runtime
-		 * assigned.
-		 */
-		if (rt_bandwidth_enabled() && rt_policy(policy) &&
-				task_group(p)->rt_bandwidth.rt_runtime == 0 &&
-				!task_group_is_autogroup(task_group(p))) {
-			task_rq_unlock(rq, p, &flags);
-			return -EPERM;
-		}
+	/*
+	 * Do not allow realtime tasks into groups that have no runtime
+	 * assigned.
+	 */
+	if (rt_policy(policy) && task_group(p)->rt_bandwidth.rt_runtime == 0 &&
+			!task_group_is_autogroup(task_group(p))) {
+		task_rq_unlock(rq, p, &flags);
+		return -EPERM;
 	}
 #endif