[Bug 197696] Implement file private locks as implemented in Linux 3.15 and submitted to the AWG for standardisation

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=197696

            Bug ID: 197696
           Summary: Implement file private locks as implemented in Linux
                    3.15 and submitted to the AWG for standardisation
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: s_bugzilla at nedprod.com

Linux kernel 3.15 adds support for file private advisory locks which are
essentially POSIX byte range advisory locks without the unfortunate per-process
first-close unlocking semantics as required by POSIX. Much more info is
available at http://lwn.net/Articles/586904/, but in essence the API is simply
new flags for fcntl():

* F_OFD_SETLK
* F_OFD_SETLKW
* F_OFD_GETLK

These still take a struct flock as before. The Linux man page describing these
is available at http://man7.org/linux/man-pages/man2/fcntl.2.html.

BSD flock() becomes a wrapper for F_ORD_SETLK with a struct flock length set to
0 (i.e. the whole file).

I'd also suggest, seeing as this is BSD, that you add asynchronous lock
notification support to kqueues.

The naive approach ought to be trivial if not hugely efficient: to EVFILT_VNODE
add a NOTE_LOCK and a NOTE_UNLOCK where kevent_data.ident is set to the fd used
to register the kevent and kevent_data.data is a pointer to a struct flock (not
sure how to bundle this through, maybe embedded via a null subsequent kevent? I
suppose the struct flock isn't absolutely needed). When the process receives a
NODE_UNLOCK kevent, it can try fcntl(F_OFD_SETLK) which if it succeeds it
initiates the changes to the file, else it goes back to waiting for the next
NODE_UNLOCK kevent to arrive.

The more sophisticated approach is to add an extra F_OFD_SETLKQ which models
EVFILT_AIO using a new EVFILT_LOCK filter. One issues F_OFD_SETLKQ with a
target kqueue to notify when the lock becomes granted. This would be a much
more efficient solution, but probably not trivial to implement.

Niall

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