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





On 07/09/2012 10:04 AM, Kevin Wolf wrote:
Am 06.07.2012 19:40, schrieb Corey Bryant:


On 07/06/2012 05:11 AM, Kevin Wolf wrote:
Am 05.07.2012 19:00, schrieb Eric Blake:
On 07/05/2012 10:35 AM, Corey Bryant wrote:
1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with
refcount of 0; fd=4's in-use flag is turned on
2. client calls 'device-add' with /dev/fdset/1 as the backing filename,
so qemu_open() increments the refcount of fdset1 to 1
3. client calls 'remove-fd fdset=1 fd=4', so qemu marks fd=4 as no
longer in-use by the monitor, and is left open because it is in use by
the block device (refcount is 1)
4. client crashes, so all tracked fds are visited; fd=4 is not in-use
but refcount is 1 so it is not closed
5. client re-establishes QMP connection, so all tracked fds are visited.
   What happens to the fd=4 in-use flag?

...but what are the semantics here?

If we _always_ turn the in-use flag back on, then that says that even
though libvirt successfully asked to close the fd, the reconnection
means that libvirt once again has to ask to close things.

If we _never_ turn the in-use flag back on, then we've broken the first
case above where we want an in-use fd to come back into use after a crash.

Maybe that argues for two flags per fd: an in-use flag (there is
currently a monitor connection that can manipulate the fd, either
because it passed in the fd or because it reconnected) and a removed
flag (a monitor called remove-fd, and no longer wants to know about the
fd, even if it crashes and reconnects).

I was in fact just going to suggest a removed flag as well, however
combined with including the monitor connections in the refcount instead
of an additional flag. This would also allow to have (the currently
mostly hypothetical case of) multiple QMP monitors without interesting
things happening.

Maybe I'm missing some point that the inuse flag would allow and
including monitors in the refcount doesn't. Is there one?

Kevin


Ok let me try this again. I was going through some of the examples and I
think we need a separate in-use flag.  Otherwise, when refcount gets to
1, we don't know if it is because of a monitor reference or a block
device reference.

Does it matter?

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.

--
Regards,
Corey



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