[3.11.y.z extended stable] Patch "sched: Fix race on toggling cfs_bandwidth_used" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    sched: Fix race on toggling cfs_bandwidth_used

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 2f811f4665af5b72415a9a1944a81692853ef07e Mon Sep 17 00:00:00 2001
From: Ben Segall <bsegall at google.com>
Date: Wed, 16 Oct 2013 11:16:12 -0700
Subject: sched: Fix race on toggling cfs_bandwidth_used

commit 1ee14e6c8cddeeb8a490d7b54cd9016e4bb900b4 upstream.

When we transition cfs_bandwidth_used to false, any currently
throttled groups will incorrectly return false from cfs_rq_throttled.
While tg_set_cfs_bandwidth will unthrottle them eventually, currently
running code (including at least dequeue_task_fair and
distribute_cfs_runtime) will cause errors.

Fix this by turning off cfs_bandwidth_used only after unthrottling all
cfs_rqs.

Tested: toggle bandwidth back and forth on a loaded cgroup. Caused
crashes in minutes without the patch, hasn't crashed with it.

Signed-off-by: Ben Segall <bsegall at google.com>
Signed-off-by: Peter Zijlstra <peterz at infradead.org>
Cc: pjt at google.com
Link: http://lkml.kernel.org/r/20131016181611.22647.80365.stgit@sword-of-the-dawn.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Cc: Chris J Arges <chris.j.arges at canonical.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/sched/core.c  |  9 ++++++++-
 kernel/sched/fair.c  | 16 +++++++++-------
 kernel/sched/sched.h |  3 ++-
 3 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 05c39f0..68924be 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7283,7 +7283,12 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)

 	runtime_enabled = quota != RUNTIME_INF;
 	runtime_was_enabled = cfs_b->quota != RUNTIME_INF;
-	account_cfs_bandwidth_used(runtime_enabled, runtime_was_enabled);
+	/*
+	 * If we need to toggle cfs_bandwidth_used, off->on must occur
+	 * before making related changes, and on->off must occur afterwards
+	 */
+	if (runtime_enabled && !runtime_was_enabled)
+		cfs_bandwidth_usage_inc();
 	raw_spin_lock_irq(&cfs_b->lock);
 	cfs_b->period = ns_to_ktime(period);
 	cfs_b->quota = quota;
@@ -7309,6 +7314,8 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 			unthrottle_cfs_rq(cfs_rq);
 		raw_spin_unlock_irq(&rq->lock);
 	}
+	if (runtime_was_enabled && !runtime_enabled)
+		cfs_bandwidth_usage_dec();
 out_unlock:
 	mutex_unlock(&cfs_constraints_mutex);

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a07a479..78a904d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2077,13 +2077,14 @@ static inline bool cfs_bandwidth_used(void)
 	return static_key_false(&__cfs_bandwidth_used);
 }

-void account_cfs_bandwidth_used(int enabled, int was_enabled)
+void cfs_bandwidth_usage_inc(void)
 {
-	/* only need to count groups transitioning between enabled/!enabled */
-	if (enabled && !was_enabled)
-		static_key_slow_inc(&__cfs_bandwidth_used);
-	else if (!enabled && was_enabled)
-		static_key_slow_dec(&__cfs_bandwidth_used);
+	static_key_slow_inc(&__cfs_bandwidth_used);
+}
+
+void cfs_bandwidth_usage_dec(void)
+{
+	static_key_slow_dec(&__cfs_bandwidth_used);
 }
 #else /* HAVE_JUMP_LABEL */
 static bool cfs_bandwidth_used(void)
@@ -2091,7 +2092,8 @@ static bool cfs_bandwidth_used(void)
 	return true;
 }

-void account_cfs_bandwidth_used(int enabled, int was_enabled) {}
+void cfs_bandwidth_usage_inc(void) {}
+void cfs_bandwidth_usage_dec(void) {}
 #endif /* HAVE_JUMP_LABEL */

 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ef0a7b2..d77f2200 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1305,7 +1305,8 @@ extern void print_rt_stats(struct seq_file *m, int cpu);
 extern void init_cfs_rq(struct cfs_rq *cfs_rq);
 extern void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq);

-extern void account_cfs_bandwidth_used(int enabled, int was_enabled);
+extern void cfs_bandwidth_usage_inc(void);
+extern void cfs_bandwidth_usage_dec(void);

 #ifdef CONFIG_NO_HZ_COMMON
 enum rq_nohz_flag_bits {
--
1.8.3.2