Re: [KVM PATCH v10] kvm: add support for irqfd

Gregory Haskins
2009-05-27T14:07:45Z

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Michael S. Tsirkin wrote:
> On Wed, May 20, 2009 at 10:30:49AM -0400, Gregory Haskins wrote:
>
>> +static int
>> +kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi)
>> +{
>> + struct _irqfd *irqfd;
>> + struct file *file = NULL;
>> + int ret;
>> +
>> + irqfd = kzalloc(sizeof(*irqfd), GFP_KERNEL);
>> + if (!irqfd)
>> + return -ENOMEM;
>> +
>> + irqfd->kvm = kvm;
>> + irqfd->gsi = gsi;
>> + INIT_LIST_HEAD(&irqfd->list);
>> + INIT_WORK(&irqfd->work, irqfd_inject);
>> +
>> + /*
>> + * Embed the file* lifetime in the irqfd.
>> + */
>> + file = fget(fd);
>> + if (IS_ERR(file)) {
>> + ret = PTR_ERR(file);
>> + goto fail;
>> + }
>>
>
> So we get a reference to a file, and unless the user is nice to us, it
> will only be dropped when kvm char device file is closed?
> I think this will deadlock if the fd in question is the open kvm char device.
>
>
>
Hmm...I hadn't considered this possibility, though I am not sure if it
would cause a deadlock in the pattern you suggest. It seems more like
it would result in, at worst, an extra reference to itself (and thus a
leak) rather than a deadlock...

I digress. In either case, perhaps I should s/fget/eventfd_fget to at
least limit the type of fd to eventfd. I was trying to be "slick" by
not needing the eventfd_fget() exported, but I am going to need to
export it later anyway for iosignalfd, so its probably a moot point.

Thanks Michael,
-Greg

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