[PATCH 5.7 146/204] sched/uclamp: Fix a deadlock when enabling uclamp static key

Greg Kroah-Hartman
2020-08-20T09:41:42Z

From: Qais Yousef <qais.yousef@arm.com>

[ Upstream commit e65855a52b479f98674998cb23b21ef5a8144b04 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 49b5c5cf9271d..ebecf1cc3b788 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1248,6 +1248,15 @@ 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;
}

@@ -1278,8 +1287,6 @@ 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);
--
2.25.1