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

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.

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

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)

7. Sometime later.. QMP disconnects; refcount for fdset is decremented to zero; fd=4 is closed