On 09/25, Peter Zijlstra wrote:
>
> On Wed, Sep 25, 2013 at 05:55:15PM +0200, Oleg Nesterov wrote:
>
> > > +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.
>
> OK, I'm not getting this; why isn't the sync_sched sufficient to get out
> of this fast path without barriers?
Aah, sorry, I didn't notice this version has another synchronize_sched()
in cpu_hotplug_done().
Then I need to recheck again...
No. Too tired too ;) damn LSB test failures...
> > > + 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.
>
> I added a goto into the cond-block, not before the cond; but see the
> version below.
"into the cond-block" doesn't look right too, at first glance. This
always succeeds, but by this time another writer can already hold
the lock.
Oleg.
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