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

[Sebastian Andrzej Siewior]

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

Sebastian