[PATCH] sched: Use RCU-sched in core-scheduling balancing logic

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

rcu_read_unlock() can incur an infrequent deadlock in
sched_core_balance(). Fix this by using the RCU-sched flavor instead.

This fixes the following spinlock recursion observed when testing the
core scheduling patches on PREEMPT=y kernel on ChromeOS:

[    3.240891] BUG: spinlock recursion on CPU#2, swapper/2/0
[    3.240900]  lock: 0xffff9cd1eeb28e40, .magic: dead4ead, .owner: swapper/2/0, .owner_cpu: 2
[    3.240905] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.4.22htcore #4
[    3.240908] Hardware name: Google Eve/Eve, BIOS Google_Eve.9584.174.0 05/29/2018
[    3.240910] Call Trace:
[    3.240919]  dump_stack+0x97/0xdb
[    3.240924]  ? spin_bug+0xa4/0xb1
[    3.240927]  do_raw_spin_lock+0x79/0x98
[    3.240931]  try_to_wake_up+0x367/0x61b
[    3.240935]  rcu_read_unlock_special+0xde/0x169
[    3.240938]  ? sched_core_balance+0xd9/0x11e
[    3.240941]  __rcu_read_unlock+0x48/0x4a
[    3.240945]  __balance_callback+0x50/0xa1
[    3.240949]  __schedule+0x55a/0x61e
[    3.240952]  schedule_idle+0x21/0x2d
[    3.240956]  do_idle+0x1d5/0x1f8
[    3.240960]  cpu_startup_entry+0x1d/0x1f
[    3.240964]  start_secondary+0x159/0x174
[    3.240967]  secondary_startup_64+0xa4/0xb0
[   14.998590] watchdog: BUG: soft lockup - CPU#0 stuck for 11s! [kworker/0:10:965]

Cc: vpillai <vpillai@digitalocean.com>
Cc: Aaron Lu <aaron.lwe@gmail.com>
Cc: Aubrey Li <aubrey.intel@gmail.com>
Cc: peterz@infradead.org
Cc: paulmck@kernel.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3045bd50e249..037e8f2e2686 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4735,7 +4735,7 @@ static void sched_core_balance(struct rq *rq)
 	struct sched_domain *sd;
 	int cpu = cpu_of(rq);
 
-	rcu_read_lock();
+	rcu_read_lock_sched();
 	raw_spin_unlock_irq(rq_lockp(rq));
 	for_each_domain(cpu, sd) {
 		if (!(sd->flags & SD_LOAD_BALANCE))
@@ -4748,7 +4748,7 @@ static void sched_core_balance(struct rq *rq)
 			break;
 	}
 	raw_spin_lock_irq(rq_lockp(rq));
-	rcu_read_unlock();
+	rcu_read_unlock_sched();
 }
 
 static DEFINE_PER_CPU(struct callback_head, core_balance_head);
-- 
2.25.1.481.gfbce0eb801-goog