Re: [PATCH v7 3/3] sched/uclamp: Fix a deadlock when enabling uclamp static key

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 24, 2020 at 10:46:50AM +0100, Qais Yousef wrote:
> On 07/24/20 11:12, Peter Zijlstra wrote:
> > On Thu, Jul 16, 2020 at 12:03:47PM +0100, Qais Yousef wrote:
> > 
> > I've trimmed the Changelog to read like:
> 
> +1
> 
> Should we mention the ordering issue too? Or maybe I misinterpreted the
> 'Possible unsafe locking scenario' part?

The lock inversion was, imo, secondary. It only existed because of the
impossible lock ordering -- taking a blocking lock inside an atomic
lock. Fixing the first, avoids the second etc.. So I left it out.

> This should work, but you'll need to sprinkle ifdef around the key. Or move it
> to uclamp_validate()

Indeed, the patch now reads like:

---
Subject: sched/uclamp: Fix a deadlock when enabling uclamp static key
From: Qais Yousef <qais.yousef@arm.com>
Date: Thu, 16 Jul 2020 12:03:47 +0100

From: 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
---
 kernel/sched/core.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1252,6 +1252,15 @@ static int uclamp_validate(struct task_s
 	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;
 }
 
@@ -1282,8 +1291,6 @@ static void __setscheduler_uclamp(struct
 	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);