Commit:     9efa68ed079af97f4e9477eadef567ffe64f7afc
Parent:     4f3b19ca41fbe572e3d44caf516c215b286fe2a6
Author:     J. Bruce Fields <[EMAIL PROTECTED]>
AuthorDate: Tue Sep 25 11:57:19 2007 -0400
Committer:  J. Bruce Fields <[EMAIL PROTECTED]>
CommitDate: Tue Oct 9 18:32:45 2007 -0400

    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 <[EMAIL PROTECTED]>
 Documentation/filesystems/mandatory-locking.txt |   21 ++++++++++++++++++++-
 1 files changed, 20 insertions(+), 1 deletions(-)

diff --git a/Documentation/filesystems/mandatory-locking.txt 
index bc449d4..0979d1d 100644
--- a/Documentation/filesystems/mandatory-locking.txt
+++ b/Documentation/filesystems/mandatory-locking.txt
@@ -3,7 +3,26 @@
                Andy Walker <[EMAIL PROTECTED]>
                           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?
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