See the following sentence in tye's scheme:

When you get [the write lock], you read from the position in the file that was the previous end of the file and update your decision as to whether you need to write.

So, in your example case, Process 321 would notice that the file changed since it last checked and that another process already sent the notification.

In reply to Re^3: avoiding a race (read locks) by Corion
in thread avoiding a race by westy032001

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