Re: __local_bh_enable_ip() vs lockdep

[Peter Zijlstra]

On Tue, Dec 15, 2020 at 02:47:24PM -0500, Steven Rostedt wrote:
> On Tue, 15 Dec 2020 20:01:52 +0100
> Heiko Carstens <hca@xxxxxxxxxxxxx> wrote:
> 
> > Hello,
> > 
> > the ftrace stack tracer kernel selftest is able to trigger the warning
> > below from time to time. This looks like there is an ordering problem
> > in __local_bh_enable_ip():
> > first there is a call to lockdep_softirqs_on() and afterwards
> > preempt_count_sub() is ftraced before it was able to modify
> > preempt_count:
> 
> Don't run ftrace stack tracer when debugging lockdep. ;-)
> 
>   /me runs!

Ha!, seriously though; that seems like something we've encountered
before, but my google-fu is failing me.

Do you remember what, if anything, was the problem with this?

---
diff --git a/kernel/softirq.c b/kernel/softirq.c
index d5bfd5e661fc..9d71046ea247 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -186,7 +186,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 	 * Keep preemption disabled until we are done with
 	 * softirq processing:
 	 */
-	preempt_count_sub(cnt - 1);
+	__preempt_count_sub(cnt - 1);
 
 	if (unlikely(!in_interrupt() && local_softirq_pending())) {
 		/*