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





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



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