Re: [libvirt] [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd
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.
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.
> 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
And in step 4 the QMP connection will increment the refcount +1 for all
fds that persisted through the QMP disconnect. (?)
I'm not sure we need the refcount increment on reconnect. 'query-fds'
should provide enough information for the new monitor to know what
fdsets are still in use by qemu, even though they are no longer
available to 'remove-fd' from the monitor, and if the monitor is worried
about keeping the fd set alive it can call 'add-fd' to again associate a
new fd with the set. The lifetime of a set is thus as long as any of
its associated fds have a non-zero refcount.
--
Eric Blake eblake(a)redhat.com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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