Re: [PATCH] cpufreq: Fix RCU reboot regression on x86 PIC machines

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Thursday, October 3, 2019 4:08:28 PM CEST Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> Since 4.20-rc1 my PIC 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. The problem
> is limited to PIC machines since they mask off interrupts
> in i8259A_shutdown() (syscore_ops.shutdown() registered
> from device_initcall()).

Let me treat this as a fresh bug report. :-)

> I reported this long ago but no better fix has surfaced,

So I don't recall seeing the original report or if I did, I had not understood
the problem then.

> hence sending out my initial workaround which I've been
> carrying around ever since. I just move cpufreq_core_init()
> to late_initcall() so the syscore_ops get registered in the
> oppsite order and thus the .shutdown() hooks get executed
> in the opposite order as well. Not 100% convinced this is
> safe (especially moving the cpufreq_global_kobject creation
> to late_initcall()) but I've not had any problems with it
> at least.

The problem is a bug in cpufreq that shouldn't point its syscore shutdown
callback pointer to cpufreq_suspend(), because the syscore stage is generally
too lat to call that function and I'm not sure why this has not been causing
any other issues to trigger (or maybe it did, but they were not reported).

Does the patch below work for you?

---
 drivers/base/core.c       |    3 +++
 drivers/cpufreq/cpufreq.c |   10 ----------
 2 files changed, 3 insertions(+), 10 deletions(-)

Index: linux-pm/drivers/cpufreq/cpufreq.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq.c
+++ linux-pm/drivers/cpufreq/cpufreq.c
@@ -2737,14 +2737,6 @@ int cpufreq_unregister_driver(struct cpu
 }
 EXPORT_SYMBOL_GPL(cpufreq_unregister_driver);
 
-/*
- * Stop cpufreq at shutdown to make sure it isn't holding any locks
- * or mutexes when secondary CPUs are halted.
- */
-static struct syscore_ops cpufreq_syscore_ops = {
-	.shutdown = cpufreq_suspend,
-};
-
 struct kobject *cpufreq_global_kobject;
 EXPORT_SYMBOL(cpufreq_global_kobject);
 
@@ -2756,8 +2748,6 @@ static int __init cpufreq_core_init(void
 	cpufreq_global_kobject = kobject_create_and_add("cpufreq", &cpu_subsys.dev_root->kobj);
 	BUG_ON(!cpufreq_global_kobject);
 
-	register_syscore_ops(&cpufreq_syscore_ops);
-
 	return 0;
 }
 module_param(off, int, 0444);
Index: linux-pm/drivers/base/core.c
===================================================================
--- linux-pm.orig/drivers/base/core.c
+++ linux-pm/drivers/base/core.c
@@ -9,6 +9,7 @@
  */
 
 #include <linux/acpi.h>
+#include <linux/cpufreq.h>
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/fwnode.h>
@@ -3179,6 +3180,8 @@ void device_shutdown(void)
 	wait_for_device_probe();
 	device_block_probing();
 
+	cpufreq_suspend();
+
 	spin_lock(&devices_kset->list_lock);
 	/*
 	 * Walk the devices list backward, shutting down each in turn.