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.