Re: [REPOST] RFC: sched: Prevent wakeup to enter critical sectionneedlessly

[Oleg Nesterov]

On 10/09, Peter Zijlstra wrote:
>
> One thing you might need to consider is the memory ordering, will the
> list_empty -- either careful or not -- observe the right list pointer,
> or could it -- when racing with wait_event()/prepare_to_wait() --
> observe a stale value. Or.. is that all already covered in on the use
> site.

I agree.

Without spin_lock(q->lock) (or some other barriers) wait_event-like
code can miss an event.

wait_event:

	prepare_to_wait(wq)	// takes wq->lock

	if (!CONDITION)
		schedule();

Now,

	CONDITION = 1;
	wake_up(wq);

at least need the full mb() before lits_empty().

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/