Re: [PATCH 0/6] sched: TTWU, IPI, and assorted stuff

[Peter Zijlstra]

On Fri, Jun 19, 2020 at 10:48:02AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 19, 2020 at 10:20:47AM -0700, Paul E. McKenney wrote:

> > > @@ -2615,7 +2617,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> > >  	 * let the waker make forward progress. This is safe because IRQs are
> > >  	 * disabled and the IPI will deliver after on_cpu is cleared.
> > >  	 */
> > > -	if (READ_ONCE(p->on_cpu) && ttwu_queue_wakelist(p, cpu, wake_flags | WF_ON_RQ))
> > > +	if (smp_load_acquire(&p->on_cpu) &&
> 
> Given the x86 memory model, this only protects against the compiler
> reordering accesses in ttwu_queue_wakelist() against the fetch of
> p->on_cpu, correct?

Yes.

> Don't get me wrong, I do see some potential compiler misorderings,
> including with cpu_rq(cpu)->nr_running.  Just curious.

Given this is arch independent code, I'd better write generic code, and
there I really think this wants to be acquire. I'll also try and write a
comment for next time.