Re: [KVM PATCH 2/3] eventfd: add a notifier mechanism

Gregory Haskins
2009-04-24T16:30:10Z

[-- Attachment #1: Type: text/plain, Size: 1709 bytes --]

Davide Libenzi wrote:
> On Fri, 24 Apr 2009, Gregory Haskins wrote:
>
>
>> Davide Libenzi wrote:
>>
>>> On Thu, 23 Apr 2009, Gregory Haskins wrote:
>>>
>>>
>>>
>>>>> Take a look at init_waitqueue_func_entry(), in particula at that "func"
>>>>> parameter. Then look at how __wake_up_common() does its thing.
>>>>> You don't need to be "waiting" for our wakeup system to work. Callbacks
>>>>> works just fine, otherwise things like epoll could not work at all.
>>>>>
>>>>>
>>>>>
>>>> Yeah, I was looking at that this afternoon after you mentioned it. That
>>>> makes sense.
>>>>
>>>> As of right now the wqh is embedded in the eventfd, accessible only by
>>>> the .read() vtable entry. In order to do this as you suggest, I imagine
>>>> I need to slightly modify the eventfd interface to allow waiters other
>>>> than the embedded readers to join the wait-queue. How would you like to
>>>> see that interface look?
>>>>
>>>>
>>> Actually, you need no changes in eventfd, since you can use its poll()
>>> directly, by dropping a callback entry.
>>> Take a look at how epoll does it in ep_insert()...
>>>
>>>
>> Ah! I just tried your suggestion and it was simple and it works. That
>> is brilliant.
>>
>> I will post a follow-up v2 that drops patch 2/3. Thanks, Davide.
>>
>
> I assume you're in control of the file/fd lifetime, so that the file*
> cannot vanish while you've your wait_queue dropped inside its
> wait_queue_head, isn't it?
>
I've addressed this issue by holding the file via eventfd_fget(), but
your review to make sure I got it right would be appreciated.

-Greg

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 266 bytes --]