Coding: Timed Event (Slideshow)

Per-Event Timestamp

  • Realtime vs. monotonic

  • jiffies is current monotonic time

  • Modify my_event

    • have a timestamp in jiffies form

    • determined in my_event_list_add()

Discussion: Deferred Events

  • ioctl() to add a deferred event

  • Can we have multiple deferred events pending?

    • ⟶ lets have only one for now. Otherwise we’d have to maintain a list somehow.

  • Who’s responsible for maintaining the timer/work?

    • ⟶ struct my_device

Add Timer

  • New ioctl: MY_INJECT_DEFERRED_EVENT, a milliseconds parameter

  • my_device_add_deferred_event()

  • Userspace: test-inject-deferred-event.c

  • Test

Gotcha: Cleanup Running Timer

  • Inject deferred event: 10s

  • rmmod my_driver

  • Wait until panic

Fix

  • my_device_destroy(): del_timer_sync()

More Gotchas

Questions

  • What if somebody injects an event while a deferred event is pending?

    • Try it out ⟶ panic

  • What if no event is pending altogether? Can I delete a timer that is not running? (Documentation is not at all clear ⟶ paranoia)

    • Try it out ⟶ work (appears to work)

Answer

  • Naive: flag and no locking

  • Bad ⟶ Spinlock (timers run in interrupt context)

Refactoring

Symmetry: my_device_add_event(), and call that from MY_INJECT_EVENT