Yeah, there is a race condition there but in that race if another process grabs the file wouldn't it then have the lock, putting the other one into wait mode? Personally I'd avoid the LOCK_SH and just go for LOCK_EX, write what you need and release (LOCK_UN). You're talking a millisecond of time really.

I once wanted to test such a thing so I created a quick module that was called by a script to lock, and release a DB database. I then proceeced to run about 6 copies of my script at once. They went crazy locking and unlocking and only had to wait for lock every few hundred goes, and even then it was only for a fraction of a second. I was all rather impressed at the time seeing 6 copies trying to grab it a few hundred times a second. Anyway, the moral is ... well you get the gist.