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

[Kevin Wolf <kwolf@redhat.com>]

Am 06.07.2012 19:40, schrieb Corey Bryant:
> 
> 
> On 07/06/2012 05:11 AM, Kevin Wolf wrote:
>> Am 05.07.2012 19:00, schrieb Eric Blake:
>>> On 07/05/2012 10:35 AM, Corey Bryant wrote:
>>>> 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with
>>>> refcount of 0; fd=4's in-use flag is turned on
>>>> 2. client calls 'device-add' with /dev/fdset/1 as the backing filename,
>>>> so qemu_open() increments the refcount of fdset1 to 1
>>>> 3. client calls 'remove-fd fdset=1 fd=4', so qemu marks fd=4 as no
>>>> longer in-use by the monitor, and is left open because it is in use by
>>>> the block device (refcount is 1)
>>>> 4. client crashes, so all tracked fds are visited; fd=4 is not in-use
>>>> but refcount is 1 so it is not closed
>>> 5. client re-establishes QMP connection, so all tracked fds are visited.
>>>   What happens to the fd=4 in-use flag?
>>>
>>> ...but what are the semantics here?
>>>
>>> If we _always_ turn the in-use flag back on, then that says that even
>>> though libvirt successfully asked to close the fd, the reconnection
>>> means that libvirt once again has to ask to close things.
>>>
>>> If we _never_ turn the in-use flag back on, then we've broken the first
>>> case above where we want an in-use fd to come back into use after a crash.
>>>
>>> Maybe that argues for two flags per fd: an in-use flag (there is
>>> currently a monitor connection that can manipulate the fd, either
>>> because it passed in the fd or because it reconnected) and a removed
>>> flag (a monitor called remove-fd, and no longer wants to know about the
>>> fd, even if it crashes and reconnects).
>>
>> I was in fact just going to suggest a removed flag as well, however
>> combined with including the monitor connections in the refcount instead
>> of an additional flag. This would also allow to have (the currently
>> mostly hypothetical case of) multiple QMP monitors without interesting
>> things happening.
>>
>> Maybe I'm missing some point that the inuse flag would allow and
>> including monitors in the refcount doesn't. Is there one?
>>
>> Kevin
>>
> 
> Ok let me try this again. I was going through some of the examples and I 
> think we need a separate in-use flag.  Otherwise, when refcount gets to 
> 1, we don't know if it is because of a monitor reference or a block 
> device reference.  

Does it matter?

> 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.

> 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.

Kevin