Why ngx_trylock do extra judgement?

Server NGINX
1

Hi there,

function ngx_shmtx_lock: if (mtx->lock == 0 &&
ngx_atomic_cmp_set(mtx->lock, 0, ngx_pid))
and ngx_trylock (ngx_atomic.h): ((lock) == 0 &&
ngx_atomic_cmp_set(lock, 0, 1))

I think ngx_atomic_cmp_set is enough, why Nginx do extra judgement ahead
of it ?