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>

1 files changed, 20 insertions, 1 deletions

diff --git a/Documentation/filesystems/mandatory-locking.txt b/Documentation/filesystems/mandatory-locking.txt
index bc449d49eee5..0979d1d2ca8b 100644
--- 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?
 ------------------------------