Re: [PATCH v8 12/15] futex: Allow to re-allocate the private local hash.

[Peter Zijlstra]

On Mon, Feb 03, 2025 at 02:59:32PM +0100, Sebastian Andrzej Siewior wrote:

> @@ -159,11 +281,25 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
>  
>  bool futex_put_private_hash(struct futex_private_hash *hb_p)
>  {
> -	return false;
> +	bool released;
> +
> +	guard(preempt)();
> +	released = rcuref_put_rcusafe(&hb_p->users);
> +	if (released)
> +		WRITE_ONCE(hb_p->released, true);
> +	return released;
>  }
>  
>  void futex_hash_put(struct futex_hash_bucket *hb)
>  {
> +	struct futex_private_hash *hb_p;
> +
> +	if (hb->hb_slot == 0)
> +		return;
> +	hb_p = container_of(hb, struct futex_private_hash,
> +			    queues[hb->hb_slot - 1]);
> +
> +	futex_put_private_hash(hb_p);
>  }
>  
>  /**
> @@ -175,6 +311,14 @@ void futex_hash_put(struct futex_hash_bucket *hb)
>   */
>  void futex_hash_get(struct futex_hash_bucket *hb)
>  {
> +	struct futex_private_hash *hb_p;
> +
> +	if (hb->hb_slot == 0)
> +		return;
> +	hb_p = container_of(hb, struct futex_private_hash,
> +			    queues[hb->hb_slot - 1]);
> +
> +	WARN_ON_ONCE(!rcuref_get(&hb_p->users));
>  }

> --- a/kernel/futex/futex.h
> +++ b/kernel/futex/futex.h
> @@ -115,6 +115,7 @@ static inline bool should_fail_futex(bool fshared)
>   */
>  struct futex_hash_bucket {
>  	atomic_t waiters;
> +	unsigned int hb_slot;
>  	spinlock_t lock;
>  	struct plist_head chain;
>  } ____cacheline_aligned_in_smp;

*groan*

Just sticking a pointer in was too complicated?

void futex_hash_put(struct futex_hash_bucket *hb)
{
	struct futex_private_hash *ph = hb->private;
	if (!ph)
		return;

	futex_put_private_hash(ph);
}