Fixed data race in flat combining algo found by TSan
Fixed data race in flat combining algo found by TSan

goswamia/libcds,hanbinhu/libcds,eugenyk/libcds,xinghuwang/libcds,hanbinhu/libcds,elventear/libcds,xinghuwang/libcds,eugenyk/libcds,goswamia/libcds,eugenyk/libcds,hanbinhu/libcds,goswamia/libcds,xinghuwang/libcds,eugenyk/libcds,xinghuwang/libcds,elventear/libcds,xinghuwang/libcds,hanbinhu/libcds,eugenyk/libcds,hanbinhu/libcds,elventear/libcds,elventear/libcds,elventear/libcds,goswamia/libcds,goswamia/libcds