Use std::atomic instead of mutex
Use std::atomic instead of mutex

hvy/chainer,chainer/chainer,hvy/chainer,jnishi/chainer,keisuke-umezawa/chainer,okuta/chainer,chainer/chainer,ktnyt/chainer,tkerola/chainer,chainer/chainer,keisuke-umezawa/chainer,niboshi/chainer,okuta/chainer,wkentaro/chainer,wkentaro/chainer,wkentaro/chainer,chainer/chainer,okuta/chainer,wkentaro/chainer,ktnyt/chainer,pfnet/chainer,ktnyt/chainer,jnishi/chainer,niboshi/chainer,ktnyt/chainer,niboshi/chainer,jnishi/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,hvy/chainer,jnishi/chainer,niboshi/chainer,hvy/chainer,okuta/chainer