Re: [PATCH v10 00/21] futex: Add support task local hash maps, FUTEX2_NUMA and FUTEX2_MPOL

[Peter Zijlstra]

On Fri, Mar 14, 2025 at 12:28:08PM +0100, Sebastian Andrzej Siewior wrote:
> On 2025-03-14 11:58:56 [+0100], Peter Zijlstra wrote:
> > On Wed, Mar 12, 2025 at 04:18:48PM +0100, Sebastian Andrzej Siewior wrote:
> > 
> > > @@ -1591,7 +1597,8 @@ static int futex_hash_allocate(unsigned int hash_slots, bool custom)
> > >  		struct futex_private_hash *free __free(kvfree) = NULL;
> > >  		struct futex_private_hash *cur, *new;
> > >  
> > > -		cur = mm->futex_phash;
> > > +		cur = rcu_dereference_protected(mm->futex_phash,
> > > +						lockdep_is_held(&mm->futex_hash_lock));
> > >  		new = mm->futex_phash_new;
> > >  		mm->futex_phash_new = NULL;
> > >  
> > 
> > Same thing again, this makes no sense.
> 
> With "mm->futex_phash" sparse complains about direct RCU access.

Yeah, but sparse is stupid.

> This makes it obvious that you can access it, it won't change as long
> as you have the lock.

It's just plain confusing. rcu_dereference() says you care about the
load being single copy atomic and the data dependency, we don't.

If we just want to shut up sparse; can't we write it like:

	cur = unrcu_pointer(mm->futex_phash);

?