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

[NeilBrown <neilb@suse.de>]

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On Sun, 27 Apr 2014 13:11:33 +0200 "Michael Kerrisk (man-pages)"
<mtk.manpages@gmail.com> wrote:

> On Sun, Apr 27, 2014 at 12:04 PM, NeilBrown <neilb@suse.de> wrote:
> > On Sun, 27 Apr 2014 11:16:02 +0200 "Michael Kerrisk (man-pages)"
> > <mtk.manpages@gmail.com> wrote:
> >
> >> [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.
> >
> > The only peculiarities I can think of are:
> >  - With NFS, locking or unlocking a region forces a flush of any cached data
> >    for that file (or maybe for the region of the file).  I'm not sure if this
> >    is worth mentioning.
> 
> I agree that it's probably not necessary to mention.
> 
> >  - With NFSv4 the client can lose a lock if it is out of contact with the
> >    server for a period of time.  When this happens, any IO to the file by a
> >    process which "thinks" it holds a lock will fail until that process closes
> >    and re-opens the file.
> >    This behaviour is since 3.12.  Prior to that the client might lose and
> >    regain the lock without ever knowing thus potentially risking corruption
> >    (but only if client and server lost contact for an extended period).
> 
> Do you have a pointer for that commit to 3.12?
> 

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ef1820f9be27b6ad158f433ab38002ab8131db4d

did most of the work while  the subsequent commit

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f6de7a39c181dfb8a2c534661a53c73afb3081cd

changed some details, added some documentation, and inverted the default
behaviour.

NeilBrown

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