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| -rw-r--r-- | doc/todo/fcntl_locks_break_with_threaded_concurrency.mdwn | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/doc/todo/fcntl_locks_break_with_threaded_concurrency.mdwn b/doc/todo/fcntl_locks_break_with_threaded_concurrency.mdwn new file mode 100644 index 000000000..c4d662723 --- /dev/null +++ b/doc/todo/fcntl_locks_break_with_threaded_concurrency.mdwn @@ -0,0 +1,27 @@ +When using git-annex get -J2, one thread can clear another thread's +transfer lock. + +This happens because fcntl locking is used, and it is not thread-safe. +<https://github.com/haskell/unix/issues/44> + +> * If a process closes any file descriptor referring to a +> file, then all of the process's locks on that file are +> released, regardless of the file descriptor(s) on which the +> locks were obtained. + +One thread starts a transfer and locks it; the second thread starts a +transfer of a different file, and in order to check annex.diskreserve, +checks to see which other transfers are currently running. In doing this +check, it closes a fd attached to the first thread's lock, which causes the +lock to be dropped. + +This only affects -J mode. + +To fix it, probably need to use STM to keep a list of transfers all threads +in the current process are doing. Then the lock checking code can avoid +re-opening locks for transfers in the STM list. + +A more generic approach is to use Annex.LockFile for everything, +and make it check its lock pool via STM for other threads holding a lock. +(Currently, the pool is only used for shared locks.) +--[[Joey]] |