[PATCH v3 0/6] locks: implement "filp-private" (aka UNPOSIX) locks

[Jeff Layton]

This patchset is the third posting of this set. Behavior between this
set and the last should be more or less the same. Here is a summary of
changes:

v3:
- more consolidation of common code between flock_to_posix_lock and
  flock_to_posix_lock64
- better bisectability by reordering changes, such that partial
  implementation won't be exposed
- s/filp/file/ s/FILP/FILE/ in symbol names

v2:
- inheritance semantics have been change to be more BSD-lock like
- patchset size has been reduced by changing how lock ownership
  is handled
- new F_UNLCKP l_type value has been added

Note too that I've gone ahead and opened a request for the POSIX folks
to consider adding this to the POSIX spec once we have something
mergeable. They seem amenable to the idea but don't want to enshrine it
into the standard until there's a real implementation of it:

    http://austingroupbugs.net/view.php?id=768

I also owe them a better writeup of the semantics for these new locks,
but have been holding off on doing that until they're a little more
settled.

As a side note, I've also had a few other userland developers reach
out to me as to the status of this work. There seems to be a lot of
interest since classic POSIX locks are such a pain to work with in
threaded programs. Hopefully some will chime in on this posting...

Original cover letter from v1 posting follows. Comments and suggestions
welcome.

-------------------------------[snip]------------------------------

At LSF this year, there was a discussion about the "wishlist" for
userland file servers. One of the things brought up was the goofy and
problematic behavior of POSIX locks when a file is closed. Boaz started
a thread on it here:

    http://permalink.gmane.org/gmane.linux.file-systems/73364

Userland fileservers often need to maintain more than one open file
descriptor on a file. The POSIX spec says:

"All locks associated with a file for a given process shall be removed
 when a file descriptor for that file is closed by that process or the
 process holding that file descriptor terminates."

This is problematic since you can't close any file descriptor without
dropping all your POSIX locks. Most userland file servers therefore
end up opening the file with more access than is really necessary, and
keeping fd's open for longer than is necessary to work around this.

This patchset is a first stab at an approach to address this problem by
adding two new l_type values -- F_RDLCKP and F_WRLCKP (the 'P' is short
for "private" -- I'm open to changing that if you have a better
mnemonic).

For all intents and purposes these lock types act just like their
"non-P" counterpart. The difference is that they are only implicitly
released when the fd against which they were acquired is closed. As a
side effect, these locks cannot be merged with "non-P" locks since they
have different semantics on close.

I've given this patchset some very basic smoke testing and it seems to
do the right thing, but it is still pretty rough. If this looks
reasonable I'll plan to do some documentation updates and will take a
stab at trying to get these new lock types added to the POSIX spec (as
HCH recommended).

At this point, my main questions are:

1) does this look useful, particularly for fileserver implementors?

2) does this look OK API-wise? We could consider different "cmd" values
   or even different syscalls, but I figured this makes it clearer that
   "P" and "non-P" locks will still conflict with one another.

Jeff Layton (6):
  locks: consolidate common code in the flock_to_posix_lock routines
  locks: consolidate checks for compatible filp->f_mode values in setlk
    handlers
  locks: rename locks_remove_flock to locks_remove_file
  locks: show private lock types in /proc/locks
  locks: report l_pid as -1 for FL_FILE_PVT locks
  locks: add new "private" lock type that is owned by the filp

 fs/file_table.c                  |   2 +-
 fs/locks.c                       | 122 +++++++++++++++++++++------------------
 include/linux/fs.h               |   5 +-
 include/uapi/asm-generic/fcntl.h |  16 +++++
 4 files changed, 85 insertions(+), 60 deletions(-)

-- 
1.8.4.2

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