From 9efa68ed079af97f4e9477eadef567ffe64f7afc Mon Sep 17 00:00:00 2001 From: "J. Bruce Fields" Date: Tue, 25 Sep 2007 11:57:19 -0400 Subject: [PATCH] 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 --- .../filesystems/mandatory-locking.txt | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) 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 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? ------------------------------