itimerdecr(): simplify code

[Scott Cheloha]

We can simplify the itimerdecr() code with the sys/time.h macros.
I think this is a lot easier to decipher.

With the macros we don't need special logic to correctly handle the
reload if the decrement exceeds the time remaining in the itimer.

With the loop we now correctly handle decrements larger than our
interval.  Maybe someone can make use of that.  Using the loop instead
of just an if-clause doesn't complicate our code so I figure we might
as well do the right thing in that case.

ok?

Index: kern_time.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_time.c,v
retrieving revision 1.120
diff -u -p -r1.120 kern_time.c
--- kern_time.c 2 Jul 2019 14:54:36 -0000       1.120
+++ kern_time.c 24 Jul 2019 23:55:21 -0000
@@ -668,45 +668,32 @@ itimerround(struct timeval *tv)
 }
 
 /*
- * Decrement an interval timer by a specified number
- * of microseconds, which must be less than a second,
- * i.e. < 1000000.  If the timer expires, then reload
- * it.  In this case, carry over (usec - old value) to
- * reduce the value reloaded into the timer so that
- * the timer does not drift.  This routine assumes
- * that it is called in a context where the timers
- * on which it is operating cannot change in value.
+ * Decrement an interval timer by the given number of microseconds.
+ * If the timer expires and it is periodic then reload it.  When reloading
+ * the timer we subtract any overrun from the next period so that the timer
+ * does not drift.
  */
 int
 itimerdecr(struct itimerval *itp, int usec)
 {
+       struct timeval decrement;
+
+       decrement.tv_sec = usec / 1000000;
+       decrement.tv_usec = usec % 1000000;
+
        mtx_enter(&itimer_mtx);
-       if (itp->it_value.tv_usec < usec) {
-               if (itp->it_value.tv_sec == 0) {
-                       /* expired, and already in next interval */
-                       usec -= itp->it_value.tv_usec;
-                       goto expire;
-               }
-               itp->it_value.tv_usec += 1000000;
-               itp->it_value.tv_sec--;
-       }
-       itp->it_value.tv_usec -= usec;
-       usec = 0;
-       if (timerisset(&itp->it_value)) {
+       timersub(&itp->it_value, &decrement, &itp->it_value);
+       if (itp->it_value.tv_sec >= 0 && timerisset(&itp->it_value)) {
                mtx_leave(&itimer_mtx);
                return (1);
        }
-       /* expired, exactly at end of interval */
-expire:
-       if (timerisset(&itp->it_interval)) {
-               itp->it_value = itp->it_interval;
-               itp->it_value.tv_usec -= usec;
-               if (itp->it_value.tv_usec < 0) {
-                       itp->it_value.tv_usec += 1000000;
-                       itp->it_value.tv_sec--;
-               }
-       } else
-               itp->it_value.tv_usec = 0;              /* sec is already 0 */
+       if (!timerisset(&itp->it_interval)) {
+               timerclear(&itp->it_value);
+               mtx_leave(&itimer_mtx);
+               return (0);
+       }
+       while (itp->it_value.tv_sec < 0 || !timerisset(&itp->it_value))
+               timeradd(&itp->it_value, &itp->it_interval, &itp->it_value);
        mtx_leave(&itimer_mtx);
        return (0);
 }