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

[Eric Blake <eblake@redhat.com>]

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On 07/03/2012 10:25 AM, Corey Bryant wrote:

>> I thought qemu would rather return the number of the fdset (which it
>> also assigns if none it passed, i.e. for fdset creation). Does libvirt
>> need the number of an individual fd?
>>
>> If libvirt prefers to assign fdset numbers itself, I'm not against it,
>> it's just something that wasn't clear to me yet.
>>
> 
> That's fine.  QEMU can return the fdset number or a string
> (/dev/fdset/1) if none is specified.  And an fdset will need to be
> specified if adding to an existing set.
> 
> I think libvirt will need the fd returned by add-fd so that it can
> evaluate fds returned by query-fd.  It's also useful for remove-fd.

Correct - since we will be adding a remove-fd, then that command needs
to know both the fdset name and the individual fd within the set to be
removed.

> 
>>> 2. drive_add file=/dev/fdset/1 -> qemu_open uses the first fd from the
>>> set that has access flags matching the qemu_open action flags.
>>> qemu_open increments refcount for this fd.
>>> 3. add-fd /dev/fdset/1 FDSET={M} -> qemu adds fd to set named
>>> "/dev/fdset/1" - command returns qemu fd to caller (e.g fd=5).  libvirt
>>> in-use flag turned on for fd.
>>> 3. block-commit -> qemu_open reopens "/dev/fdset/1" by using the first
>>> fd from the set that has access flags matching the qemu_open action
>>> flags.  qemu_open increments refcount for this fd.
>>> 4. remove-fd /dev/fdset/1 5 -> caller requests fd==5 be removed from the
>>> set.  turns libvirt in-use flag off marking the fd ready to be closed
>>> when qemu is done with it.
>>
>> If we decided to not return the individual fd numbers to libvirt, file
>> descriptors would be uniquely identified by an fdset/flags pair here.
>>
> 
> Are you saying we'd pass the fdset name and flags parameters on
> remove-fd to somehow identify the fds to remove?

Passing the flag parameters is not trivial, as that would mean the QMP
code would have to define constants mapping to all of the O_* flags that
qemu_open supports.  It's easier to support closing by fd number.

> 
>>> 5. qemu_close decrements refcount for fd, and closes fd when refcount is
>>> zero and libvirt in use flag is off.
>>
>> The monitor could just hold another reference, then we save the
>> additional flag. But that's a qemu implementation detail.
>>
> 
> I'm not sure I understand what you mean.

pass-fd (or add-fd, whatever name we give it) adds an fd to an fdset,
with initial use count of 1 (the use is the monitor).  qemu_open()
increments the use count.  A new qemu_close() wrapper would decrement
the use count.  And both calling 'remove-fd', or closing the QMP monitor
of an fd that has not yet been passed through 'remove-fd', serves as a
way to decrement the use count.  You'd still have to track whether the
monitor is using an fd (to avoid over-decrementing on QMP monitor
close), but by having the monitor's use also tracked under the refcount,
then refcount reaching 0 is sufficient to auto-close an fd.  I think
that also means that re-establishing the client QMP connection would
increment  For some examples:

1. client calls 'add-fd', qemu is now tracking fd=4 with refcount 1, in
use by monitor, as member of fdset1
2. client crashes, so all tracked fds are visited; fd=4 had not yet been
passed to 'remove-fd', so qemu decrements refcount; refcount of fd=4 is
now 0 so qemu closes it

1. client calls 'add-fd', qemu is now tracking fd=4 with refcount 1, in
use by monitor, as member of fdset1
2. client calls 'device-add' with /dev/fdset/1 as the backing filename,
so qemu_open() increments the refcount to 2
3. client crashes, so all tracked fds are visited; fd=4 had not yet been
passed to 'remove-fd', so qemu decrements refcount to 1, but leaves fd=4
open because it is still in use by the block device
4. client re-establishes QMP connection, and 'query-fds' lets client
learn about fd=4 still being open as part of fdset1, but also informs
client that fd is not in use by the monitor

1. client calls 'add-fd', qemu is now tracking fd=4 with refcount 1, in
use by monitor, as member of fdset1
2. client calls 'device-add' with /dev/fdset/1 as the backing filename,
so qemu_open() increments the refcount to 2
3. client calls 'remove-fd fdset=1 fd=4', so qemu marks fd=4 as no
longer in use by the monitor, refcount decremented to 1 but still left
open because it is in use by the block device
4. client crashes, so all tracked fds are visited; but fd=4 is already
marked as not in use by the monitor, so its refcount is unchanged

1. client calls 'add-fd', qemu is now tracking fd=4 with refcount 1, in
use by monitor, as member of fdset1
2. client calls 'device-add' with /dev/fdset/1 as the backing filename,
but the command fails for some other reason, so the refcount is still 1
at the end of the command (although it may have been temporarily
incremented then decremented during the command)
3. client calls 'remove-fd fdset=1 fd=4' to deal with the failure (or
QMP connection is closed), so qemu marks fd=4 as no longer in use by the
monitor, refcount is now decremented to 0 and fd=4 is closed

I think that covers the idea; you need a bool in_use for tracking
monitor state (the monitor is in use until either a remove-fd or a
monitor connection closes), as well as a ref-count.

>> We also need a query-fdsets command that lists all fdsets that exist. If
>> we add information about single fds to the return value of it, we
>> probably don't need a separate query-fd that operates on a single fdset.
>>
> 
> Yes, good point.  And maybe we don't need 2 commands.  query-fdsets
> could return all the sets and all the fds that are in those sets.

Yes, I think a single query command is good enough here, something like:

{ "execute":"query-fdsets" } =>
{ "return" : { "sets": [
   { "name": "fdset1",
     "fds": [ { "fd": 4, "monitor": true, "refcount": 1 } ] },
   { "name": "fdset2",
     "fds": [ { "fd": 5, "monitor": false, "refcount": 1 },
              { "fd": 6, "monitor": true, "refcount": 2 } ] } ] } }


>> In use by whom? If it's still in use in qemu (as in "in-use flag would
>> be set") and we have a refcount of zero, then that's a bug.
>>
> 
> In use by qemu.  I don't think it's a bug.  I think there are situations
> where refcount gets to zero but qemu is still using the fd.

I think the refcount being non-zero _is_ what defines an fd as being in
use by qemu (including use by the monitor).  Any place you have to close
an fd before reopening it is dangerous; the safe way is always to open
with the new permissions before closing the old permissions.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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