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

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

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.  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
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)
7. Sometime later.. QMP disconnects; refcount for fdset is decremented 
to zero; fd=4 is closed

In the following case, we have an in-use and remove flag per fd and we 
only increment/decrement refcount on qemu_open/qemu_close:

1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with
refcount of 0; fd=4's remove flag is initialized to off and in-use flag 
is initialized to on
2. client calls 'device-add' with /dev/fdset/1 as the backing filename;
qemu_open() increments the refcount of fdset1 to 1
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; fd=4's in-use flag is 
turned off; fd=4 is left open because it is still in-use by the block 
device (refcount is still 1)
4. client re-establishes QMP connection, refcount for fdset1 is still 1; 
fd=4's in-use flag is turned on; '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 1
6. qemu_close is called for fd=4; refcount for fdset1 is decremented to 
0; fd=4 is closed because (refcount == 0 && (!inuse || removed)) is true
7. Sometime later.. QMP disconnects

-- 
Regards,
Corey