This ticket originates from a question on the wiredtiger-users mailing list, and is to better document how concurrent access to BTree data structures (pages, refs, extent lists) are made thread-safe.

Some example topics to cover are the following. This list is non-exhaustive.

• Ref locks and how they behave
• How page splits are performed
• How eviction and checkpoint walk the btree in different directions (bottom up and top down respectively) to avoid races where both access the same pages in parallel
• How our skiplists allow for safe concurrent insertion

Once complete the documentation should be added to the https://source.wiredtiger.com/develop/arch-toc-int-wt-dev.html page.