Re: [PATCH -next 1/8] rcu: Fix rcu_read_unlock() deadloop due to softirq

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Jan 02, 2026 at 07:41:38PM -0500, Joel Fernandes wrote:
> 
> 
> On 1/2/2026 2:51 PM, Paul E. McKenney wrote:
> > On Fri, Jan 02, 2026 at 12:30:09PM -0500, Steven Rostedt wrote:
> >> On Fri, 2 Jan 2026 12:28:07 -0500
> >> Steven Rostedt <rostedt@goodmis.org> wrote:
> >>
> >>> Stacktrace should have recursion protection too.
> >>>
> >>> Can you try this patch to see if it would have fixed the problem too?
> >>
> >> As I believe the recursion protection should be in the tracing
> >> infrastructure more than in RCU. As RCU is used as an active participant in
> >> the kernel whereas tracing is supposed to be only an observer.
> >>
> >> If tracing is the culprit, it should be the one that is fixed.
> > 
> > Makes sense to me!  But then it would...  ;-)
> > 
> Could we fix it in both? (RCU and tracing). The patch just adds 3 more net lines
> to RCU code. It'd be good to have a guard rail against softirq recursion in RCU
> read unlock path, as much as the existing guard rail we already have with
> irq_work? After all, both paths attempt to do deferred work when it is safer to
> do so.
> 

Agreed. First it's crucial that RCU itself can prevent indefinitely
entering rcu_read_unlock_special(), because although unlikely, any RCU
reader in raise_softirq_irqoff() would cause a similar infinite loop.
Second, with solely the tracing fix, there still exists a call chain:

	rcu_read_unlock_special():
	  raise_softirq_irqoff():
	    trace_softirq_raise():
	      rcu_read_unlock_special():
	        raise_softirq_irqoff():
		  trace_softirq_raise(); // <- recursion ends here

while with the RCU fix added, the call chain ends at the second
rcu_read_unlock_special():

	rcu_read_unlock_special():
	  raise_softirq_irqoff():
	    trace_softirq_raise():
	      rcu_read_unlock_special(); // <- recursion ends here

which would slightly improve the performance becasue of fewer calls.

I'm going to include this into the RCU PR for 7.0 if no one objects.
Thanks!

Regards,
Boqun

> Yao, if you could test Steve's patch and reply whether it fixes it too?
> 
> thanks,
> 
>  - Joel
> 
> 
> 
>