File locking (of the Haskell 98 variety) was previously done using a static table with linear search, which had two problems: the array had a fixed size and was sometimes too small (#1109), and performance of lockFile/unlockFile was suboptimal due to the linear search. Also the algorithm failed to count readers as required by Haskell 98 (#629). Now it's done using a hash table (provided by the RTS). Furthermore I avoided the extra fstat() for every open file by passing the dev_t and ino_t into lockFile. This and the improvements to the locking algorithm result in a healthy 20% or so performance increase for opening/closing files (see openFile008 test).