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SubjectRe: [patch] sched: fix set_task_cpu() and provide an unlocked runqueue variant
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On Thu, 2009-11-26 at 10:35 +0100, Peter Zijlstra wrote:
> On Thu, 2009-11-26 at 02:31 +0100, Mike Galbraith wrote:
> > On Thu, 2009-11-26 at 02:01 +0100, Mike Galbraith wrote:
> > > On Wed, 2009-11-25 at 19:27 +0100, Peter Zijlstra wrote:
> >
> > > > I've got to ask, what's that barrier for?
> > >
> > > It's a leftover from frustrated bug hunting.
> >
> > To be more specific, I put it there to ensure that min_vruntimes are
> > stable.
>
> min_vruntime should only ever be poked at when holding the respective
> rq->lock, even with a barrier a 64bit read on a 32bit machine can go all
> funny.

Yeah, but we're looking at an unlocked runqueue. But never mind...

> > I figured that if try_to_wake_up() needs a barrier to look at
> > task->state, I had better do the same for the runqueues.
>
> Ah, ttwu() has that barrier for another reason. The wmb in ttwu() is to
> ensure the wakee sees the state of the waker at the time of waking.
>
> That is, its about ordering things like:
>
>
> A B
>
>
> my_cond = true;
> wake_process(my_friend);
>
> while (!my_cond)
> schedule();
>
>
> So that you can actually observe my_cond being true once you wakeup
> (schedule acts as a mb() when it actually schedules).

(Ah! I think you actually made a wrinkle in grey-ware)

...ATM, I kinda wish I'd not gone off and read barriers.txt. It hasn't
sunk in yet, but certainly has made me _paranoid as hell_ :-)

-Mike



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