Post Syndicated from Lennart Poettering original https://0pointer.net/blog/projects/locking2.html

I forgot to mention another central problem in my blog story about file locking
on Linux:

Different machines have access to different features of the same file
system. Here’s an example: let’s say you have two machines in your home LAN.
You want them to share their $HOME directory, so that you (or your family) can
use either machine and have access to all your (or their) data. So you export
/home on one machine via NFS and mount it from the other machine.

So far so good. But what happens to file locking now? Programs on the first
machine see a fully-featured ext3 or ext4 file system, where all kinds of
locking works (even though the API might suck as mentioned in the earlier blog
story). But what about the other machine? If you set up lockd properly
then POSIX locking will work on both. If you didn’t one machine can use POSIX
locking properly, the other cannot. And it gets even worse: as mentioned recent
NFS implementations on Linux transparently convert client-side BSD locking into
POSIX locking on the server side. Now, if the same application uses BSD locking on both
the client and the server side from two instances they will end up with two
orthogonal locks and although both sides think they have properly acquired a
lock (and they actually did) they will overwrite each other’s data, because
those two locks are independent. (And one wonders why the NFS developers
implemented this brokenness nonetheless…).

This basically means that locking cannot be used unless it is verified that
everyone accessing a file system can make use of the same file system feature
set. If you use file locking on a file system you should do so only if you are
sufficiently sure that nobody using a broken or weird NFS implementation might
want to access and lock those files as well. And practically that is
impossible. Even if fpathconf() was improved so that it could inform
the caller whether it can successfully apply a file lock to a file, this would
still not give any hint if the same is true for everybody else accessing the
file. But that is essential when speaking of advisory (i.e. cooperative) file
locking.

And no, this isn’t easy to fix. So again, the recommendation: forget about
file locking on Linux, it’s nothing more than a useless toy.

Also read Jeremy
Allison’s (Samba) take on POSIX file locking. It’s an interesting read.