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? 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. 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.
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.
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
Kevin