Re: [KVM PATCH v5 0/4] irqfd fixes and enhancements

[Avi Kivity <avi@redhat.com>]

On 06/25/2009 04:59 PM, Gregory Haskins wrote:
> Gregory Haskins wrote:
>    
>> (Applies to kvm.git/master:4631e094)
>>
>> The following is the latest attempt to fix the races in irqfd/eventfd, as
>> well as restore DEASSIGN support.  For more details, please read the patch
>> headers.
>>
>> This series has been tested against the kvm-eventfd unit test, and
>> appears to be functioning properly.  You can download this test here:
>>
>> ftp://ftp.novell.com/dev/ghaskins/kvm-eventfd.tar.bz2
>>
>> I've included version 4 of Davide's eventfd patch (ported to kvm.git) so
>> that its a complete reviewable series.  Note, however, that there may be
>> later versions of his patch to consider for merging, so we should
>> coordinate with him.
>>
>>      
>
> So I know we talked yesterday in the review session about whether it was
> actually worth all this complexity to deal with the POLLHUP or if we
> should just revert to the prior "two syscall" model and be done with
> it.  Rusty reflected these same sentiments this morning in response to
> Davide's patch in a different thread.
>
> I am a bit torn myself, tbh.  I do feel as though I have a good handle
> on the issue and that it is indeed now fixed (at least, if this series
> is applied and the slow-work issue is fixed, still pending upstream
> ACK).  I have a lot invested in going the POLLHUP direction having spent
> so much time thinking about the problem and working on the patches, so I
> a bit of a biased opinion, I know.
>
> The reason why I am pushing this series out now is at least partly so we
> can tie up these loose ends.  We have both solutions in front of us and
> can make a decision either way.  At least the solution is formally
> documented in the internet archives forever this way ;)
>
> I took the review comments to heart that the shutdown code was
> substantially larger and more complex than the actual fast-path code.  I
> went though last night and simplified and clarified it.  I think the
> latest result is leaner and clearer, so please give it another review
> (particularly for races) before dismissing it.
>    

Yes, it's much nicer.  I can't say I'm certain it's race free but it's a 
lot more digestible

> Ultimately, I think the concept of a release notification for eventfd is
> a good thing for all eventfd users, so I don't think this thing should
> go away per se even if irqfd decides to not use it.
>    

I agree that we want POLLHUP support, it's better than holding on to the 
eventfd.  But I think we can make it even cleaner by merging it with 
deassign.  Basically, when we get POLLHUP, we launch a slow_work (or 
something) that does a regular deassign.  That slow_work can grab a ref 
to the vm, so we don't race with the VM disappearing.

But given that the current slow_work does almost nothing, I'm not sure 
it's worth it.

-- 
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