mention open file descriptor locks

Author: vladak

synchro.tex

@@ -425,6 +425,10 @@
 update, and then calls \texttt{getpwnam}(3) to retrieve a record,
 the lock will be lost because \texttt{getpwnam}(3) opens, reads, and
 closes the password database.}
+
+Linux contains a different kind of locking called \emph{Open file description
+locks} that avoids this problem (the per file descriptor locks are released
+only on the last close), however does not provide deadlock detection.
 \item The \texttt{lockf} function (SUSv3) is simpler variant of
 \texttt{fcntl}, specifies only how to lock and how many bytes from the current
 position in the file. Very often implemented as \texttt{fcntl} wrapper.