locks: add warning about mandatory locking races

The mandatory file locking implementation has long-standing races that
probably render it useless.  I know of no plans to fix them.  Till we
do, we should at least warn people.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

diff --git a/Documentation/filesystems/mandatory-locking.txt b/Documentation/filesystems/mandatory-locking.txt
index bc449d4..0979d1d 100644 (file)
--- a/Documentation/filesystems/mandatory-locking.txt
+++ b/Documentation/filesystems/mandatory-locking.txt
@@ -3,7 +3,26 @@
                Andy Walker <andy@lysaker.kvaerner.no>
 
                           15 April 1996
-
+                    (Updated September 2007)
+
+0. Why you should avoid mandatory locking
+-----------------------------------------
+
+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?
 ------------------------------