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

[Aaro Koskinen]

Hi,

On Tue, Feb 05, 2019 at 09:05:59PM +0800, Tom Li wrote:
> On Tue, Feb 05, 2019 at 11:58:09AM +0200, Aaro Koskinen wrote:
> > Can you try below fix? It works on my Loongson.
> 
> Hello Aaro, thanks for your response. But in case you've missed
> the original thread, please check it at:
> 
> https://lkml.org/lkml/2018/11/13/857

OK, thanks. This looks slightly different from the Loongson problem:

- In Loongson, we don't get stuck in RCU, but in
  cpufreq_dbs_governor_stop -> irq_work_sync(). 

- I run non-preemptible kernel, and my system still gets stuck.

What is common is that it's UP with i8259 PIC.

> My problem is NOT about how to fix the problem on Loongson (or
> x86): the patch in the original thread (only has one-line-of-code,
> simply changes timing of cpufreq_core_init), or your patch, is
> indeed working. But they are workarounds, the real issue is the race
> condition in cpufreq.

Looking at irq_work_sync(), I cannot think how it could work on UP
machine with interrupts disabled.

A.