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    SubjectRe: [PATCH] hotplug: Optimize {get,put}_online_cpus()
    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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