[PATCH RESEND 2/2] sched/uclamp: Fix locking around cpu_util_update_eff()

[Qais Yousef <qais.yousef@arm.com>]

cpu_cgroup_css_online() calls cpu_util_update_eff() without holding the
uclamp_mutex or rcu_read_lock() like other call sites, which is
a mistake.

The uclamp_mutex is required to protect against concurrent reads and
writes that could update the cgroup hierarchy.

The rcu_read_lock() is required to traverse the cgroup data structures
in cpu_util_update_eff().

Surround the caller with the required locks and add some asserts to
better document the dependency in cpu_util_update_eff().

Fixes: 7226017ad37a ("sched/uclamp: Fix a bug in propagating uclamp value in new cgroups")
Reported-by: Quentin Perret <qperret@google.com>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---

There was no real failure observed because of this. Quentin just observed the
oddity and reported it.

 kernel/sched/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b61329299379..efa15287d09e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8690,7 +8690,11 @@ static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
 
 #ifdef CONFIG_UCLAMP_TASK_GROUP
 	/* Propagate the effective uclamp value for the new group */
+	mutex_lock(&uclamp_mutex);
+	rcu_read_lock();
 	cpu_util_update_eff(css);
+	rcu_read_unlock();
+	mutex_unlock(&uclamp_mutex);
 #endif
 
 	return 0;
@@ -8780,6 +8784,9 @@ static void cpu_util_update_eff(struct cgroup_subsys_state *css)
 	enum uclamp_id clamp_id;
 	unsigned int clamps;
 
+	lockdep_assert_held(&uclamp_mutex);
+	SCHED_WARN_ON(!rcu_read_lock_held());
+
 	css_for_each_descendant_pre(css, top_css) {
 		uc_parent = css_tg(css)->parent
 			? css_tg(css)->parent->uclamp : NULL;
-- 
2.25.1