tc_release_table is critical section of table cache. Among other things, it checks if table cache threshold is reached and if so evicts to-be-released table. This is done by comparing tc_count with tc_size.
tc_count can be atomic and checked beyond of LOCK_open. Besides it is last global variable protected by this lock.
The cost is as following:
tc_add_table() assumed that there is nothing to evict if tc_count > tc_size and skipped expensive LRU lookup. This assumption will be broken and LRU lookup must be done if tc_count >= tc_size.
A patch for this task hasn't been benchmarked yet.