flock() and NFS [Was: Re: [PATCH] locks: rename file-private locks to file-description locks]

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

[Trimming some folk from CC, and adding various NFS people]

On 04/27/2014 06:51 AM, NeilBrown wrote:

[...]

> Note to Michael: The text
>    flock() does not lock files over NFS.
> in flock(2) is no longer accurate.  The reality is ... complex.
> See nfs(5), and search for "local_lock".

Ahhh -- I see: 
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2

Thanks for the heads up.

Just in general, it would be great if the flock(2) and fcntl(2) man pages
contained correct details for NFS, of course. So, for example, if there 
are any current gotchas for NFS and fcntl() byte-range locking, I'd like 
to add those to the fcntl(2) man page.

Anyway, returning to your point about flock(), how would this text
look for the flock(2) manual page:

    NOTES
       Since  kernel  2.0,  flock() is implemented as a system call in
       its own right rather than being emulated in the GNU  C  library
       as  a  call  to fcntl(2).  This yields classical BSD semantics:
       there is no interaction between the types  of  lock  placed  by
       flock()  and  fcntl(2),  and  flock() does not detect deadlock.
       (Note, however, that on some modern BSDs, flock() and  fcntl(2)
       locks do interact with one another.)

       In Linux kernels up to 2.6.11, flock() does not lock files over
       NFS (i.e., the scope of locks was limited to the local system).
       Instead,  one could use fcntl(2) byte-range locking, which does
       work over NFS, given a sufficiently recent version of Linux and
       a  server  which  supports  locking.   Since  Linux 2.6.12, NFS
       clients support flock() locks by emulating them  as  byte-range
       locks on the entire file.  This means that fcntl(2) and flock()
       locks do interact with  one  another  over  NFS.   Since  Linux
       2.6.37,  the  kernel  supports a compatibility mode that allows
       flock() locks (and also  fcntl(2)  byte  region  locks)  to  be
       treated  as  local; see the discussion of the local_lock option
       in nfs(5).
?

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/