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