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SubjectRe: [PATCH] Convert idr's internal locking to _irqsave variant
On Thu, 13 Jul 2006 14:03:21 -0700
Roland Dreier <rdreier@cisco.com> wrote:

> > 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).
>

Good point, a try-again loop would work. Do we really need the caller to
maintain a cache? I suspect something like

drat:
if (idr_pre_get(GFP_KERNEL) == ENOMEM)
give_up();
spin_lock();
ret = idr_get_new();
spin_unlock();
if (ret == ENOMEM)
goto drat;
would do it.
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