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

[Peter Zijlstra]

On Wed, Mar 12, 2025 at 04:18:48PM +0100, Sebastian Andrzej Siewior wrote:
> @@ -196,12 +196,12 @@ static bool __futex_pivot_hash(struct mm_struct *mm,
>  {
>  	struct futex_private_hash *fph;
>  
> -	lockdep_assert_held(&mm->futex_hash_lock);
>  	WARN_ON_ONCE(mm->futex_phash_new);
>  
> -	fph = mm->futex_phash;
> +	fph = rcu_dereference_protected(mm->futex_phash,
> +					lockdep_is_held(&mm->futex_hash_lock));

I are confused... this makes no sense. Why ?!

We only ever write that variable while holding this lock, we hold the
lock, we don't need RCU to read the variable.