On Tue, 22 Apr 2014 10:10:52 -0700
[email protected] wrote:
> This is all expected behavior, and the somewhat less than useful trace
> events are expected. A task setting p->state to TASK_RUNNING without
> locks is fine if and only p == current. The standard deschedule loop is
Sure, and if you are not current, then all you need is the rq lock of
the rq that p's CPU is for.
> basically:
>
> while (1) {
> set_current_state(TASK_(UN)INTERRUPTIBLE);
Yep, and set_current_state() implies a memory barrier.
> if (should_still_sleep)
> schedule();
> }
> set_current_state(TASK_RUNNING);
The above can use __set_current_state() as there's no races to deal
with when setting current's state to RUNNING.
>
> Which can produce this in a race.
>
> The only problem this causes is a wasted check_preempt_curr call in the
> racing case, and a somewhat inaccurate sched:sched_wakeup trace event.
> Note that even if you did recheck in ttwu_do_wakeup you could still race
> and get an "inaccurate" trace event. Heck, even if the ttwu is
> _necessary_ because p is currently trying to take rq->lock to
> deschedule, you won't get a matching sched_switch event, because the
> ttwu is running before schedule is.
>
> You could sorta fix this I guess by tracking every write to p->state
> with trace events, but that would be a somewhat different change, and
> might be considered too expensive for all I know (and the trace events
> could /still/ be resolved in a different order across cpus compared to
> p->state's memory).
Yeah, let's not do that.
-- Steve
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
