[PATCH 2/2] locks: add warning about mandatory locking races

J. Bruce Fields
2007-09-25T16:56:52Z

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

diff --git a/Documentation/filesystems/mandatory-locking.txt b/Documentation/filesystems/mandatory-locking.txt
index bc449d4..8ac5cfb 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 should I 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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