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| -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 bc449d49eee..0979d1d2ca8 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? ------------------------------ |