On Mon, Sep 23, 2013 at 10:50:17AM -0400, Steven Rostedt wrote:
> On Thu, 19 Sep 2013 16:32:41 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>
> > +extern void __get_online_cpus(void);
> > +
> > +static inline void get_online_cpus(void)
> > +{
> > + might_sleep();
> > +
> > + preempt_disable();
> > + if (likely(!__cpuhp_writer || __cpuhp_writer == current))
> > + this_cpu_inc(__cpuhp_refcount);
> > + else
> > + __get_online_cpus();
> > + preempt_enable();
> > +}
>
>
> This isn't much different than srcu_read_lock(). What about doing
> something like this:
>
> static inline void get_online_cpus(void)
> {
> might_sleep();
>
> srcu_read_lock(&cpuhp_srcu);
> if (unlikely(__cpuhp_writer || __cpuhp_writer != current)) {
> srcu_read_unlock(&cpuhp_srcu);
> __get_online_cpus();
> current->online_cpus_held++;
> }
> }
There's a full memory barrier in srcu_read_lock(), while there was no
such thing in the previous fast path.
Also, why current->online_cpus_held()? That would make the write side
O(nr_tasks) instead of O(nr_cpus).
> static inline void put_online_cpus(void)
> {
> if (unlikely(current->online_cpus_held)) {
> current->online_cpus_held--;
> __put_online_cpus();
> return;
> }
>
> srcu_read_unlock(&cpuhp_srcu);
> }
Also, you might not have noticed but, srcu_read_{,un}lock() have an
extra idx thing to pass about. That doesn't fit with the hotplug api.
>
> Then have the writer simply do:
>
> __cpuhp_write = current;
> synchronize_srcu(&cpuhp_srcu);
>
> <grab the mutex here>
How does that do reader preference?
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