The acquisition of node pointer page latches is protected by index->lock latch.Currently, index->lock protected all node pointer pages either in S or X mode, and no individual block->lock were acquired on node pointer pages. With this fix, node pointer pages are protected by individual block->lock, S-latch or X-latch. The acquisition of node pointer page latches is covered by index->lock, for preventing deadlocks.
InnoDB internally uses rw-lock implementation to keep consistency of internal
resources. Basically the rw-lock has 2 types S-lock (shared) and X-lock (exluded).
The fix adds the new type SX-lock (shared excluded) for room to optimize
concurrency and improve scalability more.
At least, S-lock and X-lock behave same, and compatible for current code. So,
nothing changed by only this fix as it is. (no functional/performance changes
The new state SX-lock will be used by the future work. (e.g. WL#6326: InnoDB:
fix index->lock contention)