Subject: Re: Debugging fcntl() file locks
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Bernd Petrovitsch <bernd@petrovitsch.priv.at>
Cc: linux-nfs@vger.kernel.org
In-Reply-To: <1274967997.14002.42.camel@thorin>
References: <1274955060.14002.40.camel@thorin>
	 <1274963686.2895.7.camel@heimdal.trondhjem.org>
	 <1274967997.14002.42.camel@thorin>
Content-Type: text/plain; charset="UTF-8"
Date: Thu, 27 May 2010 10:18:47 -0400
Message-ID: <1274969927.2895.19.camel@heimdal.trondhjem.org>
Sender: linux-nfs-owner@vger.kernel.org
MIME-Version: 1.0

On Thu, 2010-05-27 at 15:46 +0200, Bernd Petrovitsch wrote:
> Hi!
> 
> On Don, 2010-05-27 at 08:34 -0400, Trond Myklebust wrote:
> > On Thu, 2010-05-27 at 12:11 +0200, Bernd Petrovitsch wrote:
> [...]
> > > How can I debug fcntl() file locks on a NFSv3-client?
> > > The server side is a NetApp-Box (no details at hand but I can ask).
> > > The client side are stock-RHEL5.3/CentOS-5.3 kernels - 2.6.18-92.el5.
> > > 
> > > The file system in question is mounted on 2 clients. flock() on a file
> > > succeeds but fcntl() fails with EAGAIN. From what I found in the
> > > Internet and manual pages, this means that someone else already locked
> > > that file (- the file is successfully open()ed read/write so it can't be
> > > "your are not allowed to write-lock the file").
> > > But how do I find out on which host and which process?
> > > 
> > > strace shows:
> > > ----  snip  ----
> > > open("/... secret ...", O_RDWR) = 3
> > > ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff7925b400) = -1 ENOTTY (Inappropriate ioctl for device)
> > > lseek(3, 0, SEEK_CUR)                   = 0
> > > fstat(3, {st_mode=S_IFREG|0600, st_size=935, ...}) = 0
> > > fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
> > > flock(3, LOCK_EX|LOCK_NB)               = 0
> > > fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=0, len=0}) = -1 EAGAIN (Resource temporarily unavailable)
> > > ----  snip  ---- 
> > > [ This is a perl script using CPAN modules at the top. ]
> [...]
> > The Linux NFS client does not allow you to lock a file using both
> > flock() and POSIX locks. You should choose one or the other locking
> > scheme. If you remove the flock() line above, then the POSIX lock will
> > likely succeed.
> 
> Thanks.
> Does flock() work on NFS these days on NFS?
> Historically that was not the case IIRC.

Yes. flock() has worked since the early 2.6.x series, however it uses
the same NFS byte range lock protocol as POSIX locks, so there is no way
to avoid conflicts.

Cheers
  Trond