Commit e65855a5 authored Jul 16, 2020 by Qais Yousef Committed by Peter Zijlstra Jul 29, 2020

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: 46609ce2 ("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

parent 21a6ee14

kernel/sched/core.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -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);

Admin message