I put this NFS race condition question to an AI and its opinion was interesting:

The race condition you are experiencing occurs because flock and NFS have historically poor compatibility. In many environments, flock only manages local locks that are invisible to other NFS clients.

To resolve this race condition, implement one of the following proven strategies:
1. Switch from flock() to fcntl()
While flock() is often local-only, fcntl() (POSIX locking) is specifically designed to work across a network via the Network Lock Manager (NLM) for NFSv3 or natively in the NFSv4 protocol. Interaction: In Linux kernels 2.6.12 and newer, flock() on an NFS mount is actually emulated using fcntl() byte-range locks, but this emulation can still be prone to race conditions during lock upgrades or if the server does not support the specific emulation. Direct implementation: Using fcntl() directly provides more reliable byte-range locking across multiple clients.

2. Use the "Link-to-Lockfile" Method (Most Reliable)
This is the most portable and robust method for NFS, as it relies on the atomicity of the link() system call, which is better supported across NFS versions than file locking. Create a unique temporary file on the same NFS filesystem (include the hostname and PID in the name). Attempt to create a hard link from this unique file to a standard "lock" filename (e.g., myfile.lock). Check the success:
If link() returns 0, the lock is acquired.
If it fails, use stat() on your unique file to see if its link count is 2; if so, you have the lock.
Release: To unlock, simply unlink() the standard lock file.

3. Verify NFS Version and Mount Options
If you must use standard locking, ensure your configuration supports it:
NFSv4: Recommended, as locking is integrated into the protocol and does not rely on external daemons like lockd or statd.
Check Mount Options: Ensure you are not using the local_lock mount option, which forces all locks to stay local to the client, effectively breaking cross-machine synchronization.
Flush Data: After releasing a lock and before another machine acquires it, ensure the data is flushed to the server to prevent the next client from reading stale, cached data.

4. Alternative: Atomic Directory Creation
On many Linux-based NFS implementations, mkdir is atomic. You can attempt to create a directory as a lock. If the operation succeeds, you hold the lock; if it returns an "already exists" error, another process has it.