Re: [PATCH 01/08] idr: add idr_replace method for replacing pointers

[Andrew Morton <akpm@osdl.org>]

Jeff Mahoney <jeffm@suse.com> wrote:
>
> +/**
> + * idr_replace - replace pointer for given id
> + * @idp: idr handle
> + * @ptr: pointer you want associated with the ide
> + * @id: lookup key
> + *
> + * Replace the pointer registered with the id.  A -ENOENT
> + * return indicates that @id was not found.
> + *
> + * The caller must serialize vs idr_find(), idr_get_new(), and idr_remove().
> + */
> +int idr_replace(struct idr *idp, void *ptr, int id)
> +{
> +	int n;
> +	struct idr_layer *p;
> +	int shift = (idp->layers - 1) * IDR_BITS;
> +
> +	n = idp->layers * IDR_BITS;
> +	p = idp->top;
> +
> +	id &= MAX_ID_MASK;
> +
> +	while ((shift > 0) && p) {
> +		n = (id >> shift) & IDR_MASK;
> +		p = p->ary[n];
> +		shift -= IDR_BITS;
> +	}
> +
> +	n = id & IDR_MASK;
> +	if (unlikely(p == NULL || !test_bit(n, &p->bitmap)))
> +		return -ENOENT;
> +
> +	p->ary[n] = ptr;
> +	return 0;
> +}

I'd have thought it would be more flexible were this to return the old
pointer.

If there was no old item, we could return NULL and "succeed".  But that gets
a bit ill-defined, because lack of an old pointer can occur if either a)
there was a layer, but the slot was empty or b) there wasn't a layer for
this new item.  So perhaps it's best to continue considering lack of an old
pointer as an error, with ERR_PTR(-ENOENT).