[tip: sched/core] sched/uclamp: Fix locking around cpu_util_update_eff()

[tip-bot2 for Qais Yousef]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     93b73858701fd01de26a4a874eb95f9b7156fd4b
Gitweb:        https://git.kernel.org/tip/93b73858701fd01de26a4a874eb95f9b7156fd4b
Author:        Qais Yousef <qais.yousef@xxxxxxx>
AuthorDate:    Mon, 10 May 2021 15:50:32 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 19 May 2021 10:53:02 +02:00

sched/uclamp: Fix locking around cpu_util_update_eff()

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@xxxxxxxxxx>
Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210510145032.1934078-3-qais.yousef@xxxxxxx
---
 kernel/sched/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f97eb73..3ec420c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9507,7 +9507,11 @@ 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;
@@ -9597,6 +9601,9 @@ 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;