Re: [RFC patch 4/7] futex: Add support for attached futexes

[Peter Zijlstra]

On Sat, Apr 02, 2016 at 11:09:18AM -0000, Thomas Gleixner wrote:
> +/**
> + * futex_detach_task - Detach task from global state
> + * @slot:    Slot number in the task local cache
> + *
> + * If the global state refcount drops to zero, the global state is destroyed.
> + */
> +static void futex_detach_task(int slot)
> +{
> +     struct futex_cache *tc = current->futex_cache;
> +     struct futex_state *fs = tc->slots[slot].fs;
> +     struct futex_hash_bucket *hb = fs->global_hb;
> +     struct futex_q *q = &fs->q;
> +
> +     /* Remove it from the task local cache */
> +     __clear_bit(slot, tc->cache_map);
> +     tc->slots[slot].uaddr = NULL;
> +     tc->slots[slot].fs = NULL;
> +
> +     /*
> +      * Lock the global hash bucket. Decrement global state refcount. If 0
> +      * remove it from the global hash and free it.
> +      */
> +     spin_lock(&hb->lock);
> +     if (--fs->refcount == 0)
> +             hb_remove_q(q, hb);
> +     else
> +             fs = NULL;
> +     spin_unlock(&hb->lock);

So you could play funny games like:

        if (atomic_add_unless(&fs->recount, -1, 1))
                return;

        spin_lock(&hb->lock);
        if (atomic_dec_return(&fs->refcount) == 0)
                hb_remove_q(q, hb);
        else
                fs = NULL;
        spin_unlock(&hb->lock);

To avoid taking that lock entirely in the 'fast' path, but I'm not sure
how performance critical this path is.

> +     kfree(fs);
> +}