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author | J. Bruce Fields <bfields@citi.umich.edu> | 2007-09-25 11:57:19 -0400 |
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committer | J. Bruce Fields <bfields@citi.umich.edu> | 2007-10-09 18:32:45 -0400 |
commit | 9efa68ed079af97f4e9477eadef567ffe64f7afc (patch) | |
tree | 4f5ea350d651f1030ae24068cdfa8b8c145167c1 /Documentation/filesystems/mandatory-locking.txt | |
parent | 4f3b19ca41fbe572e3d44caf516c215b286fe2a6 (diff) | |
download | linux-stable-9efa68ed079af97f4e9477eadef567ffe64f7afc.tar.gz linux-stable-9efa68ed079af97f4e9477eadef567ffe64f7afc.tar.bz2 linux-stable-9efa68ed079af97f4e9477eadef567ffe64f7afc.zip |
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>
Diffstat (limited to 'Documentation/filesystems/mandatory-locking.txt')
-rw-r--r-- | 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 bc449d49eee5..0979d1d2ca8b 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 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? ------------------------------ |