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locks: add warning about mandatory locking races

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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>
This commit is contained in:
J. Bruce Fields 2007-09-25 11:57:19 -04:00
parent 4f3b19ca41
commit 9efa68ed07
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19
Documentation/filesystems/mandatory-locking.txt
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@ -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?
------------------------------