Re: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp_count field

[Oleg Nesterov]

On 05/10, Paul E. McKenney wrote:
>
> On Fri, May 10, 2024 at 01:31:49PM +0200, Oleg Nesterov wrote:
>
> > Why is that?
>
> Because I run KCSAN on RCU using Kconfig options that cause KCSAN
> to be more strict.

Yes, I see now.

> > but how can KCSAN detect that all accesses to X are properly marked? I see nothing
> > KCSAN-related in the definition of WRITE_ONCE() or READ_ONCE().
>
> The trick is that KCSAN sees the volatile cast that both READ_ONCE()
> and WRITE_ONCE() use.

Hmm. grep-grep-grep... I seem to understand, DEFINE_TSAN_VOLATILE_READ_WRITE.
So __tsan_volatile_readX() will use KCSAN_ACCESS_ATOMIC.

Thanks!

> > And what does the "all accesses" above actually mean? The 2nd version does
> >
> > 	WRITE_ONCE(X, X+1);
> >
> > but "X + 1" is the plain/unmarked access?
>
> ...
>
> In that case, the "X+1" cannot be involved in a data race, so KCSAN
> won't complain.

Yes, yes, I understand now.

Paul, thanks for your explanations! and sorry for wasting your time by
provoking the unnecessarily long discussion.

I am going to send the trivial patch which moves these WARN_ON()'s under
spin_lock(), this looks more clean to me. But I won't argue if you prefer
your original patch.

Oleg.