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

[Jeff Layton]

On Tue, 29 Apr 2014 11:53:40 +0200
"Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> wrote:

> On 04/29/2014 11:24 AM, NeilBrown wrote:
> > On Tue, 29 Apr 2014 11:07:16 +0200 "Michael Kerrisk (man-pages)"
> > <mtk.manpages@xxxxxxxxx> wrote:
> > 
> >> On 04/27/2014 11:28 PM, NeilBrown wrote:
> >>> On Sun, 27 Apr 2014 13:11:33 +0200 "Michael Kerrisk (man-pages)"
> >>> <mtk.manpages@xxxxxxxxx> wrote:
> >>>
> >>>> On Sun, Apr 27, 2014 at 12:04 PM, NeilBrown <neilb@xxxxxxx> wrote:
> >>>>> On Sun, 27 Apr 2014 11:16:02 +0200 "Michael Kerrisk (man-pages)"
> >>>>> <mtk.manpages@xxxxxxxxx> 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.
> >>
> >> Thanks for that detail. What do you think of the following text for the 
> >> fcntl(2) man page:
> >>
> >>        Before  Linux 3.12, if an NFS client is out of contact with the
> >>        server for a period of time, it might lose and  regain  a  lock
> >>        without  ever  being  aware  of the fact.  This scenario potenâ
> >>        tially risks  data  corruption,  since  another  process  might
> >>        acquire  a lock in the intervening period and perform file I/O.
> >>        Since Linux 3.12, if the client loses contact with the  server,
> >>        any I/O to the file by a process which "thinks" it holds a lock
> >>        will fail until that process closes and reopens  the  file.   A
> >>        kernel  parameter,  nfs.recover_lost_locks,  can be set to 1 to
> >>        obtain the pre-3.12 behavior, whereby the client  will  attempt
> >>        to  recover  lost  locks when contact is reestablished with the
> >>        server.  Because of the attendant risk of data corruption, this
> >>        parameter defaults to 0 (disabled).
> >>
> > 
> > Mostly good.
> > 
> > I'm just a little concerned about "if the client loses contact with the
> > server" in the middle there.  It is no longer qualified and it isn't clear
> > that the "for a period of time" qualification still applied.  And we should
> > probably quantify the period of time - which defaults to 90 seconds.
> > I don't remember just now the difference between
> >    /proc/fs/nfsd/nfsv4{lease,grace}time
> > but this 90 seconds is one of those.
> > 
> > Also this is NFSv4 specific.  With NFSv3 the failure mode is the reverse.  If
> > the server loses contact with a client then any lock stays in place
> > indefinitely ("why can't I read my mail"... I remember it well).
> > 
> >   Before Linux 3.12, if an NFSv4 client loses contact with the server
> >   (defined as more than 90 seconds with no communication), it might lose
> >   and regain ....
> 
> Thanks, Neil. Changed as you suggest. I'd quite like to mention
> which of /proc/fs/nfsd/nfsv4{lease,grace}time is relevant here. I had a 
> quick scan, but could not determine it with complete confidence. My suspicion, 
> looking at fs/lockd/svcproc.c and fs/lockd/grace.c::locks_in_grace()
> is that it is /proc/fs/nfsd/nfsv4gracetime that is relevant here. Can anyone
> confirm?
> 

The difference here is subtle. The gracetime is how long after a reboot
should knfsd allow clients to reclaim state (and deny the creation of
new locks and opens). The leasetime is how long the NFSv4 lease period
is. There is a relationship between the two that's illustrated in the
comments above write_gracetime:

/**
 * write_gracetime - Set or report current NFSv4 grace period time
 *
 * As above, but sets the time of the NFSv4 grace period.
 *
 * Note this should never be set to less than the *previous*
 * lease-period time, but we don't try to enforce this.  (In the common
 * case (a new boot), we don't know what the previous lease time was
 * anyway.)
 */

The value you're interested in here is the nfsv4leasetime. If the
client doesn't renew its lease within that period, then it's subject to
the server giving up on it and dropping any state that it holds on that
clients' behalf.

Note that this is not a firm timeout. The server runs a job
periodically to clean out expired stateful objects, and it's likely
that there is some time (maybe even up to another whole lease period)
between when the timeout expires and the job actually runs. If the
client gets a RENEW in there within that window, its lease will be
renewed and its state preserved.

Also note that all of the above just applies to the Linux knfsd. There
are many other servers in the field and they have different rules for
dropping state held by clients that have gone AWOL.

-- 
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/