Re: [PATCH] Convert idr's internal locking to _irqsave variant

[Roland Dreier <rdreier@cisco.com>]

 > I suspect it'll get really ugly.  It's a container library which needs to
 > allocate memory when items are added, like the radix-tree.  Either it needs
 > to assume GFP_ATOMIC, which is bad and can easily fail or it does weird
 > things like radix_tree_preload().

Actually I don't think it has to be too bad.  We could tweak the
interface a little bit so that consumers do something like:

	struct idr_layer *layer = NULL;	/* opaque */

retry:
        spin_lock(&my_idr_lock);
	ret = idr_get_new(&my_idr, ptr, &id, layer);
        spin_unlock(&my_idr_lock);

        if (ret == -EAGAIN) {
		layer = idr_alloc_layer(&my_idr, GFP_KERNEL);
		if (!IS_ERR(layer))
			goto retry;
	}

in other words make the consumer responsible for passing in new memory
that can be used for a new entry (or freed if other entries have
become free in the meantime).

 - R.