[tip: sched/core] sched/uclamp: Fix a deadlock when enabling uclamp static key

["tip-bot2 for Qais Yousef" <tip-bot2@linutronix.de>]

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

Commit-ID:     e65855a52b479f98674998cb23b21ef5a8144b04
Gitweb:        https://git.kernel.org/tip/e65855a52b479f98674998cb23b21ef5a8144b04
Author:        Qais Yousef <qais.yousef@arm.com>
AuthorDate:    Thu, 16 Jul 2020 12:03:47 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 29 Jul 2020 13:51:47 +02:00

sched/uclamp: Fix a deadlock when enabling uclamp static key

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
---
 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 6782534..e44d83f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1346,6 +1346,15 @@ 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;
 }
 
@@ -1376,8 +1385,6 @@ 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);