Well I'm assuming that you don't want to sleep if, say,
ep->eventcnt is non-zero. The code is currently (simplified):
add_wait_queue(...);
if (ep->eventcnt)
break;
/* window here */
set_current_state(TASK_INTERRUPTIBLE);
schedule();
If another CPU increments eventcnt and sends this task a wakeup in that
window, it is missed and we still sleep. The conventional fix for that
is:
add_wait_queue(...);
set_current_state(TASK_INTERRUPTIBLE);
if (ep->eventcnt)
break;
/* harmless window here */
schedule();
So if someone delivers a wakeup in the "harmless window" then this task
still calls schedule(), but the wakeup has turned the state from
TASK_INTERRUPTIBLE into TASK_RUNNING, so the schedule() doesn't actually
take this task off the runqueue. This task will zoom straight through the
schedule() and will then loop back and notice the incremented ep->eventcnt.
So it is important that the waker increment eventcnt _before_ delivering
the wake_up, too.
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