Re: CFS rq lock question

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2008-04-11 at 19:37 +0200, Peter Zijlstra wrote:
> On Fri, 2008-04-11 at 13:21 -0400, Dan Upton wrote:
> > I'm poking around with some scheduler stuff, and there's something I'm
> > not clear on for the CFS runqueue locks.  The comments before
> > __load_balance_iterator(...) in sched_fair.c suggests things can be
> > dequeued even though the runqueue lock is held.  Can things also be
> > added to the queue while the lock is held?  (Also, either way, what's
> > the rationale that dequeueing is a safe procedure when somebody else
> > holds a lock?)
> 
> /*
>  * Load-balancing iterator. Note: while the runqueue stays locked
>  * during the whole iteration, the current task might be
>  * dequeued so the iterator has to be dequeue-safe. Here we
>  * achieve that by always pre-iterating before returning
>  * the current task:
>  */
> 
> I don't think this comment is correct, but if it were, it would only
> apply to rq->curr, not for any enqueue/dequeue.
> 

D'oh, I'm silly..

the task can be dequeued because we move it to another cpu.