CatalogCache::_scheduleCollectionRefresh holds CatalogCache::_mutex while calling ShardServerCatalogCacheLoader::getsChunkSince, which will try to grab ShardServerCCL::_mutex at the beginning:
https://github.com/mongodb/mongo/blob/r4.0.10/src/mongo/s/catalog_cache.cpp#L603

Inside async task, tries to run callback while holding ShardServerCLL::_mutex:
https://github.com/mongodb/mongo/blob/r4.0.10/src/mongo/db/s/shard_server_catalog_cache_loader.cpp#L416

and the callback tries to grab the CatalogCache::_mutex:
https://github.com/mongodb/mongo/blob/r4.0.10/src/mongo/s/catalog_cache.cpp#L577