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Subject[PATCH 1/5] sched: Fix race on toggling cfs_bandwidth_used
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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@google.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 0c3feeb..2d2ac236 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7433,7 +7433,12 @@ 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;
@@ -7459,6 +7464,8 @@ 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 4aa0b10..67c9ecf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2854,13 +2854,14 @@ 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)
@@ -2868,7 +2869,8 @@ 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 d69cb32..d73761f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1334,7 +1334,8 @@ 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 {


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