Re: [Qemu-devel] [PATCH v5 6/6] block: Enable qemu_open/close to work with fd sets

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

On 07/25/2012 11:57 PM, Corey Bryant wrote:
>
>
> On 07/25/2012 03:43 PM, Eric Blake wrote:
>> On 07/23/2012 07:08 AM, Corey Bryant wrote:
>>> When qemu_open is passed a filename of the "/dev/fdset/nnn"
>>> format (where nnn is the fdset ID), an fd with matching access
>>> mode flags will be searched for within the specified monitor
>>> fd set.  If the fd is found, a dup of the fd will be returned
>>> from qemu_open.
>>>
>>> Each fd set has a reference count.  The purpose of the reference
>>> count is to determine if an fd set contains file descriptors that
>>> have open dup() references that have not yet been closed.  It is
>>> incremented on qemu_open and decremented on qemu_close.  It is
>>> not until the refcount is zero that file desriptors in an fd set
>>> can be closed.  If an fd set has dup() references open, then we
>>> must keep the other fds in the fd set open in case a reopen
>>> of the file occurs that requires an fd with a different access
>>> mode.
>>>
>>
>>> +++ b/monitor.c
>>> @@ -2551,6 +2551,91 @@ static void monitor_fdsets_set_in_use(Monitor
>>> *mon, bool in_use)
>>>       }
>>>   }
>>>
>>> +void monitor_fdset_increment_refcount(Monitor *mon, int64_t fdset_id)
>>> +{
>>> +    mon_fdset_t *mon_fdset;
>>> +
>>> +    if (!mon) {
>>> +        return;
>>> +    }
>>
>> Am I reading this code right by stating that 'if there is no monitor, we
>> don't increment the refcount'?  How does a monitor reattach affect
>> things?  Or am I missing something fundamental about the cases when
>> 'mon==NULL' will exist?
>>
>
> Yes you're reading this correctly.
>
> I'm pretty sure that mon will only be NULL if QEMU is started without a
> monitor.
>
> If QEMU has a monitor, and libvirt disconnects it's connection to the
> qemu monitor, then I believe mon will remain non-NULL.

I've verified this to be true and everything is working as expected.

If libvirt's connection to the monitor fd is closed, mon will remain 
non-NULL and the refcount will still be incremented/decremented on 
qemu_open()/qemu_close().  When libvirt reconnects, any fdsets that 
haven't been cleaned up will still be available.  query-fdsets can be 
used to determine what's available.

-- 
Regards,
Corey