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

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

On 07/09/2012 10:05 AM, Luiz Capitulino wrote:
> On Thu, 05 Jul 2012 11:06:56 -0400
> Corey Bryant <coreyb@linux.vnet.ibm.com> wrote:
>
>>
>>
>> On 07/04/2012 04:09 AM, Kevin Wolf wrote:
>>> 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.
>>
>> Ok.  So refcount would be incremented (for the fd or fdset, whatever we
>> decide on) when QMP reconnects.  I'm assuming we wouldn't wait until
>> after a query-fds call.
>
> I'm not sure this is a good idea because we will leak fds if the client forgets
> about the fds when re-connecting (ie. it was restarted) or if a different
> client connects to QMP.
>
> If we really want to do that, I think that the right way of doing this is to
> add a command for clients to re-again ownership of the fds on reconnection.
>
> But to be honest, I don't fully understand why this is needed.
>

I'm not sure this is an issue with current design.  I know things have 
changed a bit as the email threads evolved, so I'll paste the current 
design that I am working from.  Please let me know if you still see any 
issues.

FD passing:
-----------
New monitor commands enable adding/removing an fd to/from a set.  New 
monitor command query-fdsets enables querying of current monitor fdsets. 
  The set of fds should all refer to the same file, with each fd having 
different access flags (ie. O_RDWR, O_RDONLY).  qemu_open can then dup 
the fd that has the matching access mode flags.

Design points:
--------------
1. add-fd
-> fd is passed via SCM rights and qemu adds fd to first unused fdset 
(e.g. /dev/fdset/1)
-> add-fd monitor function initializes the monitor inuse flag for the 
fdset to true
-> add-fd monitor function initializes the remove flag for the fd to false
-> add-fd returns fdset number and received fd number (e.g fd=3) to caller

2. drive_add file=/dev/fdset/1
-> qemu_open uses the first fd in fdset1 that has access flags matching 
the qemu_open action flags and has remove flag set to false
-> qemu_open increments refcount for the fdset
-> Need to make sure that if a command like 'device-add' fails that 
refcount is not incremented

3. add-fd fdset=1
-> fd is passed via SCM rights
-> add-fd monitor function adds the received fd to the specified fdset 
(or fails if fdset doesn't exist)
-> add-fd monitor function initializes the remove flag for the fd to false
-> add-fd returns fdset number and received fd number (e.g fd=4) to caller

4. block-commit
-> qemu_open performs "reopen" by using the first fd from the fdset that 
has access flags matching the qemu_open action flags and has remove flag 
set to false
-> qemu_open increments refcount for the fdset
-> Need to make sure that if a command like 'block-commit' fails that 
refcount is not incremented

5. remove-fd fdset=1 fd=4
-> remove-fd monitor function fails if fdset doesn't exist
-> remove-fd monitor function turns on remove flag for fd=4

6. qemu_close (need to replace all close calls in block layer with 
qemu_close)
-> qemu_close decrements refcount for fdset
-> qemu_close closes all fds that have (refcount == 0 && (!inuse || remove))
-> qemu_close frees the fdset if no fds remain in it

7. disconnecting the QMP monitor
-> monitor disconnect visits all fdsets on monitor and turns off monitor 
in-use flag for fdset

8. connecting the QMP monitor
-> monitor connect visits all fdsets on monitor and turns on monitor 
in-use flag for fdset

9. query-fdsets
-> returns all fdsets and fds that don't have remove flag on

QMP command examples
--------------------
-> { "execute": "add-fd", "arguments": { "fdset": 1 } }
<- { "return": { "fdset": 1, "fd": 3 } }

-> { "execute": "remove-fd", "arguments": { "fdset": 1, "fd": 3 } }
<- { "return": {} }

-> { "execute":"query-fdsets" } =>
<- { "return" : { "fdsets": [
      { "name": "fdset1",
        "fds": [ { "fd": 4, "removed": false } ],
        "refcount": 1,
        "monitor": true },
      { "name": "fdset2",
        "fds": [ { "fd": 5, "removed": false },
                 { "fd": 6, "removed": true } ],
        "refcount": 1,
        "monitor": true } }

-- 
Regards,
Corey