[Barrelfish-users] One-shot timer: deadline already elapsed

[Chothia Zaheer]

Hello again,

With oneshot_timer enabled, this assertion would regularly trip on nos6:
  kernel 0: ** arch_set_timer: t = 10319, kernel_now = 10453
  kernel 0 PANIC! kernel assertion "t > kernel_now" failed at ../kernel/arch/x86/timing.c:257

The code essentially assumes that kernel_now will not change during this sequence:
  schedule() -> update_timer() -> arch_set_timer()

However, here is a scenario where that doesn't hold (occurs every few seconds):
* Setup a timer for the next deadline at t = 10319 which is in the future.
  (In scheduler_rbed, line 386 with kernel_now = 10239, wcet = 80, etime = 0.)
* In the interim, an APIC timer interrupt arrives and kernel_now is updated to
  t = 10453.
* update_timer resumes with timer = 10319 but that has already elapsed.

A simple fix is to ignore expired timer requests, but I don't know if this might
violate assumptions of the client code:

diff --git a/kernel/timer.c b/kernel/timer.c
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -31,7 +31,7 @@
 {
     systime_t timer = MIN(next_sched_timer, next_wakeup_timer);
     assert(timer != 0);
-    if (last_timer != timer) {
+    if (last_timer != timer && timer > kernel_now) {
         #if 0 /* does not seem to create a problem */
         if (timer == TIMER_INF) {
             printk(LOG_WARN, "********* %s:%s() timer == TIMER_INF\n", __FILE__, __FUNCTION__);

Regards,

--Zaheer