Am 09.07.2012 17:23, schrieb Corey Bryant:
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.
Ah right... So this would only work if we had explicit fdset-create/close commands, where the former would increase the refcount and the latter decrease it (fdset-open would be optional but I like symmetry) Maybe we need (or want) that anyway, but I need to think more about it first.
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.
Ok, I'll have a look. Kevin