Re: Testing of shared RCU branching

[Joel Fernandes <joelagnelf@nvidia.com>]

On Tue, Feb 25, 2025 at 09:54:29AM -0800, Paul E. McKenney wrote:
> On Tue, Feb 25, 2025 at 08:11:11AM -0800, Boqun Feng wrote:
[...]
> > > > 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.
> 
> Here you go!  And you are right, if it is a regression, it is from a
> long time ago, though something more recent might have made it more
> probable.

In my opinion I probably wouldn't even call it a regression because the
data-race is happening on a boolean element. If I am not mistaken, this is
thus a false-positive and KCSAN has no way of silencing it?

thanks,

 - Joel