diff --git a/doc/forum/git-annex_parallel_invocation_and_the_git-annex_branch/comment_4_aec9399cdf7b11011c9b577a6800b490._comment b/doc/forum/git-annex_parallel_invocation_and_the_git-annex_branch/comment_4_aec9399cdf7b11011c9b577a6800b490._comment
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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2019-05-06T18:46:21Z"
+ content="""
+[[todo/only_allow_one_git_queue_to_be_flushed_at_a_time]]
+"""]]
diff --git a/doc/todo/only_allow_one_git_queue_to_be_flushed_at_a_time.mdwn b/doc/todo/only_allow_one_git_queue_to_be_flushed_at_a_time.mdwn
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+A git queue is used when doing things that update .git/index.
+If two git-annex processes are both building up a git queue and happen
+to flush at the same time, one will fail due to git's locking of the index
+file.
+
+This doesn't affect multiple threads with --jobs; while threads have
+individual git queues, inter-thread locking allows only one to flush
+at a time. That locking is handled in `Annex.Queue.flush'` using a
+semaphore.
+
+Seems that it would be better to use a lock file. --[[Joey]]