Bug#54914: InnoDB: performance drop with innodb_change_buffering=all

Reduce ibuf_mutex and ibuf_pessimistic_insert_mutex contention further.

Protect ibuf->empty by the insert buffer root page latch, not ibuf_mutex.

ibuf_tree_root_get(): Assert that ibuf_mutex is owned by the
caller. Assert that the stamped page number is correct. Assert that
ibuf->empty agrees with the root page.

ibuf_size_update(): Do not update ibuf->empty.

ibuf_init_at_db_start(): Update ibuf->empty while holding the root page latch.

ibuf_add_free_page(): Return TRUE/FALSE instead of DB_SUCCESS/DB_STRONG_FAIL.

ibuf_remove_free_page(): Release ibuf_pessimistic_insert_mutex as
early as possible.

ibuf_contract_ext(): Rely on a dirty read of ibuf->empty, unless the
server is being shut down. Never acquire ibuf_mutex. Eliminate n_stored.

ibuf_contract_after_insert(): Never acquire ibuf_mutex. Perform dirty
reads of ibuf->size and ibuf->max_size.

ibuf_insert_low(): Only acquire ibuf_mutex for mode==BTR_MODIFY_TREE.
Perform dirty reads of ibuf->size and ibuf->max_size. Update
ibuf->empty while holding the root page latch.

ibuf_delete_rec(): Update ibuf->empty while holding the root page latch.

ibuf_is_empty(): Release ibuf_mutex earlier.

storage/innobase/include/ibuf0ibuf.ic

@@ -46,11 +46,12 @@ struct ibuf_struct{
 	ulint		seg_size;	/*!< allocated pages of the file
 					segment containing ibuf header and
 					tree */
-	ibool		empty;		/*!< after an insert to the ibuf tree
-					is performed, this is set to FALSE,
-					and if a contract operation finds
-					the tree empty, this is set to
-					TRUE */
+	ibool		empty;		/*!< Protected by the page
+					latch of the root page of the
+					insert buffer tree
+					(FSP_IBUF_TREE_ROOT_PAGE_NO). TRUE
+					if and only if the insert
+					buffer tree is empty. */
 	ulint		free_list_len;	/*!< length of the free list */
 	ulint		height;		/*!< tree height */
 	dict_index_t*	index;		/*!< insert buffer index */