[Xen-devel] Re: [PATCH 1/3] xen/event: Add reference counting to event channels

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2011-10-26 at 16:47 +0100, Daniel De Graaf wrote:
> @@ -939,6 +943,10 @@ static void unbind_from_irq(unsigned int irq)
>  {
>         struct evtchn_close close;
>         int evtchn = evtchn_from_irq(irq);
> +       struct irq_info *info = irq_get_handler_data(irq);
> +
> +       if (atomic_read(&info->refcnt) > 0 && 
> !atomic_dec_and_test(&info->refcnt))
> +               return;

This isn't all that atomic any more...

evtchn_make_refcounted() doesn't seem to have any locking which would
save you...

Perhaps you could always manipulate this flag under the mapping lock
(which perhaps is normally taken around about the sort of place you'd
want to do this anyway) and make it non-atomic?

Or maybe you could build something with cmpxchg?

Ian.


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