/*
 * Note that 'init' is a special process: it doesn't get signals it doesn't
 * want to handle. Thus you cannot kill init even with a SIGKILL even by
 * mistake.
 *
 * Note that we go through the signals twice: once to check the signals that
 * the kernel can handle, and then we build all the user-level signal handling
 * stack-frames in one go after that.
 */
static int do_signal(sigset_t *oldset, struct pt_regs *regs, int syscall)
{
 struct k_sigaction ka;
 siginfo_t info;
 int signr;

 /*
  * We want the common case to go fast, which
  * is why we may in certain cases get here from
  * kernel mode. Just return without doing anything
  * if so.
  */
 if (!user_mode(regs))//regs保存的是进入内核态之前的寄存器现场，必须为用户模式，否则直接返回
  return 0;

 if (try_to_freeze())
  goto no_signal;

 if (current->ptrace & PT_SINGLESTEP)
  ptrace_cancel_bpt(current);//和调试相关，我们在后面的文章中会具体分析

 signr = get_signal_to_deliver(&info, &ka, regs, NULL);//取出等待处理的信号
 if (signr > 0) {
  handle_signal(signr, &ka, &info, oldset, regs, syscall);//处理信号
  if (current->ptrace & PT_SINGLESTEP)
   ptrace_set_bpt(current);
  return 1;
 }

 no_signal:
 /*
  * No signal to deliver to the process - restart the syscall.
  */
 if (syscall) {
  if (regs->ARM_r0 == -ERESTART_RESTARTBLOCK) {
   if (thumb_mode(regs)) {
    regs->ARM_r7 = __NR_restart_syscall;
    regs->ARM_pc -= 2;
   } else {
    u32 __user *usp;

    regs->ARM_sp -= 12;
    usp = (u32 __user *)regs->ARM_sp;

    put_user(regs->ARM_pc, &usp[0]);
    /* swi __NR_restart_syscall */
    put_user(0xef000000 | __NR_restart_syscall, &usp[1]);
    /* ldr pc, [sp], #12 */
    put_user(0xe49df00c, &usp[2]);

    flush_icache_range((unsigned long)usp,
         (unsigned long)(usp + 3));

    regs->ARM_pc = regs->ARM_sp + 4;
   }
  }
  if (regs->ARM_r0 == -ERESTARTNOHAND ||
      regs->ARM_r0 == -ERESTARTSYS ||
      regs->ARM_r0 == -ERESTARTNOINTR) {
   restart_syscall(regs);
  }
 }
 if (current->ptrace & PT_SINGLESTEP)
  ptrace_set_bpt(current);
 return 0;
}