locks: add warning about mandatory locking races

		Andy Walker <andy@lysaker.kvaerner.no>

			   15 April 1996

		     (Updated September 2007)

0. Why you should avoid mandatory locking

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The Linux implementation is prey to a number of difficult-to-fix race

conditions which in practice make it not dependable:

	- The write system call checks for a mandatory lock only once

	  at its start.  It is therefore possible for a lock request to

	  be granted after this check but before the data is modified.

	  A process may then see file data change even while a mandatory

	  lock was held.

	- Similarly, an exclusive lock may be granted on a file after

	  the kernel has decided to proceed with a read, but before the

	  read has actually completed, and the reading process may see

	  the file data in a state which should not have been visible

	  to it.

	- Similar races make the claimed mutual exclusion between lock

	  and mmap similarly unreliable.

1. What is  mandatory locking?

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