Re: Testing of shared RCU branching

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

On Tue, Feb 25, 2025 at 07:58:01AM -0800, Paul E. McKenney wrote:
> Hello, Boqun,
> 
> I have run overnight tests on your earlier branches here:
> 
> ccb986e8b69f ("MAINTAINERS: Update Joel's email address")
> 
> These passed other than a KCSAN complaint involving
> rcu_preempt_deferred_qs_handler() and rcu_read_unlock_special().
> This looks like the plain C-language writes to ->defer_qs_iw_pending.
> 
> My guess is that this is low probability, despite having happened twice,
> and that it happens when rcu_read_unlock_special() is interrupted,
> resulting in rcu_preempt_deferred_qs_handler() being invoked as an
> IRQ-work handler.  Keeping in mind that RCU runs KCSAN so as to locate
> data races between task and handler on the same CPU.
> 
> Thoughts?
> 

Do you have a KCSAN of this? Also this is not a regression, right?
Meaning you probably have seen this before? Anyway, it should be an easy
fix (just using READ_ONCE() and WRITE_ONCE()). I can send the fix out
and put it in.

Regards,
Boqun

> 							Thanx, Paul