Re: [patch] sched: fix set_task_cpu() and provide an unlocked runqueue variant

[Peter Zijlstra <peterz@infradead.org>]

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.

>  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).