On Fri, Sep 27, 2013 at 10:41:16PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 27, 2013 at 08:15:32PM +0200, Oleg Nesterov wrote:
> > On 09/26, Peter Zijlstra wrote:
[ . . . ]
> > > +static bool cpuhp_readers_active_check(void)
> > > {
> > > + unsigned int seq = per_cpu_sum(cpuhp_seq);
> > > +
> > > + smp_mb(); /* B matches A */
> > > +
> > > + /*
> > > + * In other words, if we see __get_online_cpus() cpuhp_seq increment,
> > > + * we are guaranteed to also see its __cpuhp_refcount increment.
> > > + */
> > >
> > > + if (per_cpu_sum(__cpuhp_refcount) != 0)
> > > + return false;
> > >
> > > + smp_mb(); /* D matches C */
> >
> > It seems that both barries could be smp_rmb() ? I am not sure the comments
> > from srcu_readers_active_idx_check() can explain mb(), note that
> > __srcu_read_lock() always succeeds unlike get_cpus_online().
>
> I see what you mean; cpuhp_readers_active_check() is all purely reads;
> there are no writes to order.
>
> Paul; is there any argument for the MB here as opposed to RMB; and if
> not should we change both these and SRCU?
Given that these memory barriers execute only on the semi-slow path,
why add the complexity of moving from smp_mb() to either smp_rmb()
or smp_wmb()? Straight smp_mb() is easier to reason about and more
robust against future changes.
Thanx, Paul
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