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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