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

>> 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 > > It also decreases the reference count because the monitor doesn't use it > any more. I don't think that will work because refcount is for the entire fdset. So we can't decrement the refcount for every fd that is removed from the fdset. I think it is much simpler if we only increment refcount for an fdset on qemu_open, and only decrement refcount on qemu_close.

>> 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) > > So here the refcount becomes 0 and the fdset is closed. > >> 7. Sometime later.. QMP disconnects; refcount for fdset is decremented >> to zero; fd=4 is closed > > The only question that is a bit unclear to me is whether a remove-fd on > one monitor drops the refcount only for this monitor or for all > monitors. However, both options can be implemented without additional > flags or counters. Before we go back and forth on this thread, would you mind taking a look at the last email I sent to Luiz? It includes all the design points that I'm currently working from. I think it's a good level set and we can work off that thread if there are still any issues.