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

Paul E. McKenney
2008-03-19T20:49:23Z

On Tue, Mar 18, 2008 at 01:42:48PM +0100, Heiko Carstens wrote:
> On Mon, Mar 17, 2008 at 01:43:57PM -0700, Paul E. McKenney wrote:
> > 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. ;-)
>
> Not sure if I can follow you here, but for the smp case cpu_set() is
> nothing but a set_bit() which implies both: a memory barrier and a compiler
> barrier on s390. The compare and swap instruction used here ensures that
> all previous memory accesses will be seen by other cpus.

That should make this work as far as I can see!

Thanx, Paul

> Btw. the code sequence above will go away soon anyway, since I'm converting
> our code to the generic NO_HZ infrastructure.
>
> > > > 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?
>
> That should never happen because of the compiler and memory barrier semantics
> of cpu_set(). Or... I'm completely misunderstanding you, which wouldn't me
> surprise me too much :)