Re: Parallel shared to exclusive flock conversion blocks forever on single NFS client

[Trond Myklebust <trondmy@hammerspace.com>]

On Wed, 2025-03-12 at 22:57 +0100, Tycho Kirchner wrote:
> Dear NFS kernel developers,
> In `man 2 flock` it is documented, that an existing lock can be 
> converted to a new lock mode. Multiple processes on the *same* client
> converting their LOCK_SH to LOCK_EX quickly results in a deadlock of
> the 
> client processes. This can already be reproduced on a single physical
> machine, with for instance the NFS server running in a VM and the
> host 
> machine connecting to it as a client.
> 
> Steps to reproduce:
> - Setup a virtual machine with Virtualbox and install NFS-server
> - Create an /etc/export: /home/VMUSER/nfs  10.0.2.2(rw,async)
> - Create a NAT firewall rule forwarding NFS port 2049 to the VM
> - Mount the export on the host, chdir it and create an empty file:
>    $ sudo mount -t nfs 127.0.0.1:/home/VMUSER/nfs  /somedir
>    $ cd /somedir
>    $ touch foo
> - Execute below attached ~/locktest.py in parallel on the client:
>    $ for i in {1..10}; do ~/locktest.py foo & done; wait
> - Wait half a minute. The command does not terminate. Ever.
> - Abort execution with Ctrl+C and kill leftovers: pkill -f
> locktest.py
> 
> Notes:
> - According to my tests, from three concurrent client-processes
> onwards, 
> the block quickly occurs.
> - Placing a `fcntl.flock(a, fcntl.LOCK_UN)` before fcntl.LOCK_EX is 
> enough, so the deadlock never occurs.
> - OR'ing `| fcntl.LOCK_NB` quickly results in endless
> »BlockingIOError« 
> exceptions with no client process making any progress. See the also 
> attached ~/locktest_NB.py.
> - Multiple distributions, Kernelversions and combinations tested,
> e.g. 
> NFS-client KVER 6.6.67 on Debian12 and KVER 6.12.17-amd64 on 
> DebianTesting, or KVER 6.4.0-150600.23.38-default on openSUSE Leap
> 15.6. 
> The error was always and quickly reproducible.
> 

The same manpage also states:

       Converting a lock (shared to exclusive, or vice versa) is not guaranteed
       to be atomic: the existing lock is first removed, and then a new lock is
       established.  Between these two steps, a pending lock request by another
       process may be granted, with  the  result  that  the  conversion  either
       blocks,  or  fails  if LOCK_NB was specified.  (This is the original BSD
       behavior, and occurs on many other implementations.)

so there is no harm in adding the LOCK_UN because you cannot expect
atomicity.

Cheers
  Trond
-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com