linux-kernel - [PATCH] sched: Don't allow priority switch to realtime when the task doesn't belong to init_task_group and when CONFIG_RT_GROUP

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Date:	Thu, 20 Nov 2008 11:48:54 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
Subject: [PATCH] sched: Don't allow priority switch to realtime when the
	task doesn't belong to init_task_group and when
	CONFIG_RT_GROUP_SCHED isn't set

Applies on 2.6.28-rc5.

With CONFIG_RT_GROUP_SCHED not set, don't allow a task's priority switch
to realtime if the task isn't part of init_task_group.

A task belonging to a fair group could use sched_setscheduler/sched_setparam
to become a realtime task. If such a task belongs to one of the
child groups of init_task_group and if CONFIG_RT_GROUP_SCHED is not set,
then it ends up getting queued in init_task_group's runqueue.
So we have a situation where, a task belongs to one group (child)
but ends in the runqueue of another group (init_task_group).
This does not look correct.

Fix this by failing such priority change requests in sched_setscheduler()
and sched_setparam().

Signed-off-by: Bharata B Rao <bharata@...ux.vnet.ibm.com>
---
 kernel/sched.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5206,6 +5206,13 @@ recheck:
 		if (rt_bandwidth_enabled() && rt_policy(policy) &&
 				task_group(p)->rt_bandwidth.rt_runtime == 0)
 			return -EPERM;
+#elif defined(CONFIG_FAIR_GROUP_SCHED)
+		/*
+		 * If the task doesn't belong to init_task_group, don't
+		 * allow priority switch to realtime. (!CONFIG_RT_GROUP_SCHED)
+		 */
+		if (rt_policy(policy) && (task_group(p) != &init_task_group))
+			return -EPERM;
 #endif

 		retval = security_task_setscheduler(p, policy, param);
Regards,
Bharata.
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