Re: [REGRESSION 4.20-rc1] 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds")

[Aaro Koskinen <aaro.koskinen@iki.fi>]

Hi,

On Tue, Feb 05, 2019 at 01:07:00PM +0800, Tom Li wrote:
> On Tue, Nov 13, 2018 at 03:54:53PM +0200, Ville Syrjälä wrote:
> > Hi Paul,
> > After 4.20-rc1 some of my 32bit UP machines no longer reboot/shutdown.
> > I bisected this down to commit 45975c7d21a1 ("rcu: Define RCU-sched
> > API in terms of RCU for Tree RCU PREEMPT builds").
> > 
> > I traced the hang into
> > -> cpufreq_suspend()
> >  -> cpufreq_stop_governor()
> >   -> cpufreq_dbs_governor_stop()
> >    -> gov_clear_update_util()
> >     -> synchronize_sched()
> >      -> synchronize_rcu()
> >
> > Only PREEMPT=y is affected for obvious reasons, but that couldn't
> > explain why the same UP kernel booted on an SMP machine worked fine.
> > Eventually I realized that the difference between working and
> > non-working machine was IOAPIC vs. PIC. With initcall_debug I saw
> > that we mask everything in the PIC before cpufreq is shut down,
> 
> Hello Paul & Ville.
> 
> I'm not a kernel hacker, just a n00b playing with various non-x86
> systems, but I've been forced getting into kernel hacking due to
> the same issue.
> 
> Since February, I'm working on porting some trivial out-of-tree
> drivers to the upstream, and noticed my Yeeloong 8089D, a machine running
> the Loongson 2F 64-bit MIPS-III processor, will completely hang during
> reboot or shutdown. I tried bisecting it for 24 hours without sleep, but
> the attempt had failed.
> 
> I managed to narrow it in-between 4.19 and 4.20, most unusual thing I've
> observed was its probabilistic nature. The chance of triggering it seems
> getting progressively higher since 4.20, making pinpointing the specific
> commit difficult, finally with a 100% chance since Linux 4.19.
> 
> I initially suspected a bug in the platform driver, later I also tried
> to disable various kernel hardening options, all without success. At this
> point I've realized, because it has shown to be probabilistic, it must be
> a race condition, and since it's an uniprocessor system, it must be the
> CPU scheduler getting preempted somehow. Disabling CONFIG_PREEMPT makes
> the issue go away. I tried to add various preempt_disable() in the platform
> driver but didn't work.
> 
> Finally I've hooked up a netconsole and started adding printk()s.
> 
> [   23.652000] loongson2_cpufreq loongson2_cpufreq: shutdown
> [   23.656000] loongson-gpio loongson-gpio: shutdown
> [   23.660000] migrate_to_reboot_cpu()
> [   23.664000] syscore_shutdown()
> [   23.668000] PM: Calling i8259A_shutdown+0x0/0xa8
> [   23.672000] PM: Calling irq_gc_shutdown+0x0/0x88
> [   23.672000] PM: Calling cpufreq_suspend+0x0/0x1a0
> [   23.672000] cpufreq_suspend()
> [   23.672000] cpufreq_suspend: Suspending Governors
> [   23.672000] cpufreq_stop_governor()
> [   23.672000] cpufreq_stop_governor: for CPU 0
> 
> Looks like something in the core cpufreq code? So I tried searching
> "cpufreq_stop_governor()" at LKML... Oops!

Can you try below fix? It works on my Loongson.

A.

---

From: Aaro Koskinen <aaro.koskinen@iki.fi>
Date: Wed, 2 Jan 2019 22:58:52 +0200
Subject: [PATCH] irqchip/i8259: fix shutdown order by moving syscore_ops
 registration

When using cpufreq on Loongson 2F MIPS platform, "poweroff"
command gets frequently stuck in syscore_shutdown(). The reason is
that i8259A_shutdown() gets called before cpufreq_suspend(), and if we
have pending work then irq_work_sync() in cpufreq_dbs_governor_stop()
gets stuck forever as we have all interrupts masked already.

irq-i8259 is registering syscore_ops using device_initcall(),
while cpufreq uses core_initcall(). Fix the shutdown order simply
by registering the irq syscore_ops during the early IRQ init instead
of using a separate initcall at later stage.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
---
 drivers/irqchip/irq-i8259.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/drivers/irqchip/irq-i8259.c b/drivers/irqchip/irq-i8259.c
index b0d4aab1a58c..d000870d9b6b 100644
--- a/drivers/irqchip/irq-i8259.c
+++ b/drivers/irqchip/irq-i8259.c
@@ -225,14 +225,6 @@ static struct syscore_ops i8259_syscore_ops = {
 	.shutdown = i8259A_shutdown,
 };
 
-static int __init i8259A_init_sysfs(void)
-{
-	register_syscore_ops(&i8259_syscore_ops);
-	return 0;
-}
-
-device_initcall(i8259A_init_sysfs);
-
 static void init_8259A(int auto_eoi)
 {
 	unsigned long flags;
@@ -332,6 +324,7 @@ struct irq_domain * __init __init_i8259_irqs(struct device_node *node)
 		panic("Failed to add i8259 IRQ domain");
 
 	setup_irq(I8259A_IRQ_BASE + PIC_CASCADE_IR, &irq2);
+	register_syscore_ops(&i8259_syscore_ops);
 	return domain;
 }
 
-- 
2.17.0