bwmf: Fix deadlock described in #61
bwmf: Fix deadlock described in #61

BaiGang/taskgraph,fengjingchao/taskgraph,kod3r/taskgraph,plutoshe/taskgraph,dzch/taskgraph,dzch/taskgraph,5kg/taskgraph,BaiGang/taskgraph,taskgraph/taskgraph,kod3r/taskgraph,plutoshe/taskgraph,taskgraph/taskgraph,plutoshe/mapreduceOfTaskgraph,plutoshe/mapreduceOfTaskgraph,5kg/taskgraph,fengjingchao/taskgraph