Re: [PATCH v10 00/21] futex: Add support task local hash maps, FUTEX2_NUMA and FUTEX2_MPOL
From: Peter Zijlstra
Date: Fri Mar 14 2025 - 07:41:21 EST
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);
?