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

[Corey Bryant]

On 07/09/2012 11:46 AM, Kevin Wolf wrote:
> Am 09.07.2012 17:05, schrieb Corey Bryant:
> > I'm not sure this is an issue with current design.  I know things have
> > changed a bit as the email threads evolved, so I'll paste the current
> > design that I am working from.  Please let me know if you still see any
> > issues.
> >
> > FD passing:
> > -----------
> > New monitor commands enable adding/removing an fd to/from a set.  New
> > monitor command query-fdsets enables querying of current monitor fdsets.
> >    The set of fds should all refer to the same file, with each fd having
> > different access flags (ie. O_RDWR, O_RDONLY).  qemu_open can then dup
> > the fd that has the matching access mode flags.
> >
> > Design points:
> > --------------
> > 1. add-fd
> > -> fd is passed via SCM rights and qemu adds fd to first unused fdset
> > (e.g. /dev/fdset/1)
> > -> add-fd monitor function initializes the monitor inuse flag for the
> > fdset to true
> > -> add-fd monitor function initializes the remove flag for the fd to false
> > -> add-fd returns fdset number and received fd number (e.g fd=3) to caller
> >
> > 2. drive_add file=/dev/fdset/1
> > -> qemu_open uses the first fd in fdset1 that has access flags matching
> > the qemu_open action flags and has remove flag set to false
> > -> qemu_open increments refcount for the fdset
> > -> Need to make sure that if a command like 'device-add' fails that
> > refcount is not incremented
> >
> > 3. add-fd fdset=1
> > -> fd is passed via SCM rights
> > -> add-fd monitor function adds the received fd to the specified fdset
> > (or fails if fdset doesn't exist)
> > -> add-fd monitor function initializes the remove flag for the fd to false
> > -> add-fd returns fdset number and received fd number (e.g fd=4) to caller
> >
> > 4. block-commit
> > -> qemu_open performs "reopen" by using the first fd from the fdset that
> > has access flags matching the qemu_open action flags and has remove flag
> > set to false
> > -> qemu_open increments refcount for the fdset
> > -> Need to make sure that if a command like 'block-commit' fails that
> > refcount is not incremented
> >
> > 5. remove-fd fdset=1 fd=4
> > -> remove-fd monitor function fails if fdset doesn't exist
> > -> remove-fd monitor function turns on remove flag for fd=4
>
> What was again the reason why we keep removed fds in the fdset at all?

Because if refcount is > 0 for the fd set, then the fd could be in use 
by a block device.  So we keep it around until refcount is decremented 
to zero, at which point it is safe to close.

> The removed flag would make sense for a fdset after a hypothetical
> close-fdset call because the fdset needs to be kept around until the
> last user closes it, but I think removed fds can be deleted immediately.

fds in an fd set really need to be kept around until zero block devices 
reference them.  At that point, if '(refcount == 0 && (!inuse || 
remove))' is true, then we'll officially close the fd.

> I think I might have confused remove-fd and close-fdset in earlier
> emails in this thread, so I hope this isn't inconsistent with what I
> said before.

Ok no problem.

> > 6. qemu_close (need to replace all close calls in block layer with
> > qemu_close)
> > -> qemu_close decrements refcount for fdset
> > -> qemu_close closes all fds that have (refcount == 0 && (!inuse || remove))
> > -> qemu_close frees the fdset if no fds remain in it
> >
> > 7. disconnecting the QMP monitor
> > -> monitor disconnect visits all fdsets on monitor and turns off monitor
> > in-use flag for fdset
>
> And close all fds with refcount == 0.

Yes, this makes sense.

It also makes sense to close removed fds with refcount == 0 in the 
remove-fd function.  Basically this will be the same thing we do in 
qemu_close.  We'll close any fds that evaulate the following as true:

(refcount == 0 && (!inuse || remove))

> > 8. connecting the QMP monitor
> > -> monitor connect visits all fdsets on monitor and turns on monitor
> > in-use flag for fdset
> >
> > 9. query-fdsets
> > -> returns all fdsets and fds that don't have remove flag on

--
Regards,
Corey