Re: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz()

Paul E. McKenney
2008-03-17T20:44:17Z

On Mon, Mar 17, 2008 at 11:17:41PM +0300, Oleg Nesterov wrote:
> (to clarify: my question is completely offtopic to this patch)
> On 03/17, Paul E. McKenney wrote:
> > On Mon, Mar 17, 2008 at 09:30:47PM +0300, Oleg Nesterov wrote:
> > > I'm not sure the code below is up to date, but what I have in
> > > arch/s390/kernel/time.c is:
> > >
> > > stop_hz_timer:
> > >
> > > cpu_set(cpu, nohz_cpu_mask);
> > >
> > > if (rcu_needs_cpu(cpu) || local_softirq_pending()) {
> > > cpu_clear(cpu, nohz_cpu_mask);
> > > return;
> > > }
> > >
> > > Don't we need smp_mb() after cpu_set() ?
> >
> > S390's memory model is quite strong, so it might not be needed.
>
> OK, in that case we shouldn't worry.

I don't know if I would go -that- far. ;-)

> > In any
> > case, if needed, it goes -before- the cpu_set(), because the problems
> > would arise if prior RCU read-side critical sections were to be reordered
> > to follow this cpu_set(), right?
>
> No, but it is very possible I missed something.
>
> What if rcu_needs_cpu(cpu) is executed before cpu_set(cpu, nohz_cpu_mask)?
> It can miss rcu_start_batch() -> rcp->cur++ and return false, but at the
> same time rcu_start_batch() may see nohz_cpu_mask without this CPU.

If you mean that the rcu_needs_cpu() executes before the cpu_set() in
the code fragment above, while the rcu_start_batch() executes on some
other CPU?

Hmmm.... Can't see why this wouldn't be a problem, right off-hand,
though I cannot claim to be an s390 expert.

Heiko, thoughts?

Thanx, Paul

> No?
>
> Oleg.
>
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