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

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Date:	Fri, 24 Apr 2009 12:29:35 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com
Subject: Re: [KVM PATCH 2/3] eventfd: add a notifier mechanism

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



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