On Wed, Sep 25, 2013 at 05:55:15PM +0200, Oleg Nesterov wrote:
> On 09/24, Peter Zijlstra wrote:
> >
> > So now we drop from a no memory barriers fast path, into a memory
> > barrier 'slow' path into blocking.
>
> Cough... can't understand the above ;) In fact I can't understand
> the patch... see below. But in any case, afaics the fast path
> needs mb() unless you add another synchronize_sched() into
> cpu_hotplug_done().
For whatever it is worth, I too don't see how it works without read-side
memory barriers.
Thanx, Paul
> > +static inline void get_online_cpus(void)
> > +{
> > + might_sleep();
> > +
> > + /* Support reader-in-reader recursion */
> > + if (current->cpuhp_ref++) {
> > + barrier();
> > + return;
> > + }
> > +
> > + preempt_disable();
> > + if (likely(!__cpuhp_writer))
> > + __this_cpu_inc(__cpuhp_refcount);
>
> mb() to ensure the reader can't miss, say, a STORE done inside
> the cpu_hotplug_begin/end section.
>
> put_online_cpus() needs mb() as well.
>
> > +void __get_online_cpus(void)
> > +{
> > + if (__cpuhp_writer == 1) {
> > + /* See __srcu_read_lock() */
> > + __this_cpu_inc(__cpuhp_refcount);
> > + smp_mb();
> > + __this_cpu_inc(cpuhp_seq);
> > + return;
> > + }
>
> OK, cpuhp_seq should guarantee cpuhp_readers_active_check() gets
> the "stable" numbers. Looks suspicious... but lets assume this
> works.
>
> However, I do not see how "__cpuhp_writer == 1" can work, please
> see below.
>
> > + /*
> > + * XXX list_empty_careful(&cpuhp_readers.task_list) ?
> > + */
> > + if (atomic_dec_and_test(&cpuhp_waitcount))
> > + wake_up_all(&cpuhp_writer);
>
> Same problem as in previous version. __get_online_cpus() succeeds
> without incrementing __cpuhp_refcount. "goto start" can't help
> afaics.
>
> > void cpu_hotplug_begin(void)
> > {
> > - cpu_hotplug.active_writer = current;
> > + unsigned int count = 0;
> > + int cpu;
> >
> > - for (;;) {
> > - mutex_lock(&cpu_hotplug.lock);
> > - if (likely(!cpu_hotplug.refcount))
> > - break;
> > - __set_current_state(TASK_UNINTERRUPTIBLE);
> > - mutex_unlock(&cpu_hotplug.lock);
> > - schedule();
> > - }
> > + lockdep_assert_held(&cpu_add_remove_lock);
> > +
> > + /* allow reader-in-writer recursion */
> > + current->cpuhp_ref++;
> > +
> > + /* make readers take the slow path */
> > + __cpuhp_writer = 1;
> > +
> > + /* See percpu_down_write() */
> > + synchronize_sched();
>
> Suppose there are no readers at this point,
>
> > +
> > + /* make readers block */
> > + __cpuhp_writer = 2;
> > +
> > + /* Wait for all readers to go away */
> > + wait_event(cpuhp_writer, cpuhp_readers_active_check());
>
> So wait_event() "quickly" returns.
>
> Now. Why the new reader should see __cpuhp_writer = 2 ? It can
> still see it == 1, and take that "if (__cpuhp_writer == 1)" path
> above.
>
> Oleg.
>
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