[PATCH 2/2] sched/uclamp: Fix a deadlock when enabling uclamp static key

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

The following splat was caught when setting uclamp value of a task:

  BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:49

   cpus_read_lock+0x68/0x130
   static_key_enable+0x1c/0x38
   __sched_setscheduler+0x900/0xad8

Fix by ensuring we enable the key outside of the critical section in
__sched_setscheduler()

Fixes: 46609ce22703 ("sched/uclamp: Protect uclamp fast path code with static key")
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200716110347.19553-4-qais.yousef@arm.com
(cherry picked from commit e65855a52b479f98674998cb23b21ef5a8144b04)
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---
 kernel/sched/core.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a8ab68aa189a..352239c411a4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1249,6 +1249,15 @@ static int uclamp_validate(struct task_struct *p,
 	if (upper_bound > SCHED_CAPACITY_SCALE)
 		return -EINVAL;
 
+	/*
+	 * We have valid uclamp attributes; make sure uclamp is enabled.
+	 *
+	 * We need to do that here, because enabling static branches is a
+	 * blocking operation which obviously cannot be done while holding
+	 * scheduler locks.
+	 */
+	static_branch_enable(&sched_uclamp_used);
+
 	return 0;
 }
 
@@ -1279,8 +1288,6 @@ static void __setscheduler_uclamp(struct task_struct *p,
 	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
 		return;
 
-	static_branch_enable(&sched_uclamp_used);
-
 	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
 		uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
 			      attr->sched_util_min, true);
-- 
2.17.1