glibc ucontext: how does setcontext set the sigmask?

Question

This is the structure of ucontext_t:

typedef struct ucontext_t
  {
    unsigned long int __ctx(uc_flags);
    struct ucontext_t *uc_link;
    stack_t uc_stack;
    mcontext_t uc_mcontext;
    sigset_t uc_sigmask;
    struct _libc_fpstate __fpregs_mem;
    __extension__ unsigned long long int __ssp[4];
  } ucontext_t;

I know uc_sigmask specifies the signals to be blocked after a setcontext. But how is it implemented? Is it atomic to set sigmask and set up registers and jmp to to the target rip?