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

[Kevin Wolf <kwolf@redhat.com>]

Am 09.07.2012 17:23, schrieb Corey Bryant:
>>> I think it would cause fds to sit on the monitor
>>> until refcount gets to zero (monitor disconnects). Here's an example
>>> without the in-use flag:
>>>
>>> 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with
>>> refcount of 1 (incremented because of monitor reference); fd=4's remove
>>> flag is initialized to off
>>> 2. client calls 'device-add' with /dev/fdset/1 as the backing filename;
>>> qemu_open() increments the refcount of fdset1 to 2
>>> 3. client crashes, so all fdsets are visited; fd=4 had not yet been
>>> passed to 'remove-fd', so it's remove flag is off; refcount for fdset1
>>> is decremented to 1; fd=4 is left open because it is still in use by the
>>> block device (refcount is 1)
>>> 4. client re-establishes QMP connection, refcount for fdset1 is
>>> incremented to 2; 'query-fds' lets client learn about fd=4 still being
>>> open as part of fdset1
>>> 5. client calls 'remove-fd fdset=1 fd=4'; qemu turns on remove flag for
>>> fd=4; but fd=4 remains open because refcount of fdset1 is 2
>>
>> It also decreases the reference count because the monitor doesn't use it
>> any more.
> 
> I don't think that will work because refcount is for the entire fdset. 
> So we can't decrement the refcount for every fd that is removed from the 
> fdset.
> 
> I think it is much simpler if we only increment refcount for an fdset on 
> qemu_open, and only decrement refcount on qemu_close.

Ah right... So this would only work if we had explicit
fdset-create/close commands, where the former would increase the
refcount and the latter decrease it (fdset-open would be optional but I
like symmetry)

Maybe we need (or want) that anyway, but I need to think more about it
first.

>>> 6. qemu_close is called for fd=4; refcount for fdset1 is decremented to
>>> 1; fd=4 remains open because monitor still references fdset1 (refcount
>>> of fdset1 is 1)
>>
>> So here the refcount becomes 0 and the fdset is closed.
>>
> 
>>> 7. Sometime later.. QMP disconnects; refcount for fdset is decremented
>>> to zero; fd=4 is closed
>>
>> The only question that is a bit unclear to me is whether a remove-fd on
>> one monitor drops the refcount only for this monitor or for all
>> monitors. However, both options can be implemented without additional
>> flags or counters.
> 
> Before we go back and forth on this thread, would you mind taking a look 
> at the last email I sent to Luiz?  It includes all the design points 
> that I'm currently working from.  I think it's a good level set and we 
> can work off that thread if there are still any issues.

Ok, I'll have a look.

Kevin