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authorJ. Bruce Fields <bfields@citi.umich.edu>2007-09-25 11:57:19 -0400
committerJ. Bruce Fields <bfields@citi.umich.edu>2007-10-09 18:32:45 -0400
commit9efa68ed079af97f4e9477eadef567ffe64f7afc (patch)
tree4f5ea350d651f1030ae24068cdfa8b8c145167c1 /Documentation/filesystems
parent4f3b19ca41fbe572e3d44caf516c215b286fe2a6 (diff)
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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>
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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?
------------------------------
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