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    SubjectRe: [PATCH] hotplug: Optimize {get,put}_online_cpus()
    On Thu, Sep 26, 2013 at 06:58:40PM +0200, Oleg Nesterov wrote:
    > Peter,
    >
    > Sorry. Unlikely I will be able to read this patch today. So let me
    > ask another potentially wrong question without any thinking.
    >
    > On 09/26, Peter Zijlstra wrote:
    > >
    > > +void __get_online_cpus(void)
    > > +{
    > > +again:
    > > + /* See __srcu_read_lock() */
    > > + __this_cpu_inc(__cpuhp_refcount);
    > > + smp_mb(); /* A matches B, E */
    > > + __this_cpu_inc(cpuhp_seq);
    > > +
    > > + if (unlikely(__cpuhp_state == readers_block)) {
    >
    > OK. Either we should see state = BLOCK or the writer should notice the
    > change in __cpuhp_refcount/seq. (altough I'd like to recheck this
    > cpuhp_seq logic ;)
    >
    > > + atomic_inc(&cpuhp_waitcount);
    > > + __put_online_cpus();
    >
    > OK, this does wake(cpuhp_writer).
    >
    > > void cpu_hotplug_begin(void)
    > > {
    > > ...
    > > + /*
    > > + * Notify new readers to block; up until now, and thus throughout the
    > > + * longish synchronize_sched() above, new readers could still come in.
    > > + */
    > > + __cpuhp_state = readers_block;
    > > +
    > > + smp_mb(); /* E matches A */
    > > +
    > > + /*
    > > + * If they don't see our writer of readers_block to __cpuhp_state,
    > > + * then we are guaranteed to see their __cpuhp_refcount increment, and
    > > + * therefore will wait for them.
    > > + */
    > > +
    > > + /* Wait for all now active readers to complete. */
    > > + wait_event(cpuhp_writer, cpuhp_readers_active_check());
    >
    > But. doesn't this mean that we need __wait_event() here as well?
    >
    > Isn't it possible that the reader sees BLOCK but the writer does _not_
    > see the change in __cpuhp_refcount/cpuhp_seq? Those mb's guarantee
    > "either", not "both".

    But if the readers does see BLOCK it will not be an active reader no
    more; and thus the writer doesn't need to observe and wait for it.

    > Don't we need to ensure that we can't check cpuhp_readers_active_check()
    > after wake(cpuhp_writer) was already called by the reader and before we
    > take the same lock?

    I'm too tired to fully grasp what you're asking here; but given the
    previous answer I think not.



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