r9 is used to determine whether syscall restarting must be performed or
not.  Unfortunately, r9 is never set to zero in the non-syscall path,
and r9 is on top of that a callee-saved register which can be set to
non-zero by the C functions that are called during IRQ handling.

This means that if r10 (used for the syscall return value) is one of the
-ERESTART* values when a hardware interrupt occurs which leads to a
signal being delivered to the process, the kernel will "restart" a
syscall which never occurred.  This will lead to the PC being moved back
by 2 on return to user space.

Fix the problem by setting r9 to zero in the interrupt path.

Test case (should loop forever but ends up executing the break 8 trap
instruction):

  #include <signal.h>
  #include <stdlib.h>
  #include <sys/time.h>

  void f(int n)
  {
  	register int r9 asm ("r9") = 1;
  	register int r10 asm ("r10") = n;

          __asm__ __volatile__(
  		"ba	1f	\n"
  		"nop		\n"
  		"break	8	\n"
  		"1: ba	.	\n"
  		"nop		\n"
  		:
  		: "r" (r9), "r" (r10)
  		: "memory");
  }

  void handler1(int sig) { }

  int main(int argc, char *argv[])
  {
          struct itimerval t1 = { .it_value = {1} };

          signal(SIGALRM, handler1);
          setitimer(ITIMER_REAL, &t1, NULL);

          f(-513); /* -ERESTARTNOINTR */

          return 0;
  }
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Jesper Nilsson <jespern@axis.com>