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Re: [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd
Date: Wed, 04 Jul 2012 10:09:06 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0

Am 03.07.2012 20:21, schrieb Corey Bryant:
> On 07/03/2012 02:00 PM, Eric Blake wrote:
>> On 07/03/2012 11:46 AM, Corey Bryant wrote:
>>> Yes, I think adding a +1 to the refcount for the monitor makes sense.
>>> I'm a bit unsure how to increment the refcount when a monitor reconnects
>>> though.  Maybe it is as simple as adding a +1 to each fd's refcount when
>>> the next QMP monitor connects.
>> Or maybe delay a +1 until after a 'query-fds' - it is not until the
>> monitor has reconnected and learned what fds it should be aware of that
>> incrementing the refcount again makes sense.  But that would mean making
>> 'query-fds' track whether this is the first call since the monitor
>> reconnected, as it shouldn't normally increase refcounts.
> This doesn't sound ideal.

Yes, it's less than ideal.

>> The other alternative is that the monitor never re-increments a
>> refcount.  Once a monitor disconnects, that fd is lost to the monitor,
>> and a reconnected monitor must pass in a new fd to be re-associated with
>> the fdset.  In other words, the monitor's use of an fd is a one-way
>> operation, starting life in use but ending at the first disconnect or
>> remove-fd.
> I would vote for this 2nd alternative.  As long as we're not introducing 
> an fd leak.  And I don't think we are if we decrement the refcount on 
> remove-fd or on QMP disconnect.

In fact, I believe this one is even worse. I can already see hacks like
adding a dummy FD with invalid flags and removing it again just to
regain control over the fdset...

You earlier suggestion made a lot of sense to me: Whenever a new QMP
monitor is connected, increase the refcount. That is, as long as any
monitor is there, don't drop any fdsets unless explicitly requested via QMP.


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