On 09/26, Peter Zijlstra wrote:
>
> On Thu, Sep 26, 2013 at 05:53:21PM +0200, Oleg Nesterov wrote:
> > On 09/26, Peter Zijlstra wrote:
> > > void cpu_hotplug_done(void)
> > > {
> > > - cpu_hotplug.active_writer = NULL;
> > > - mutex_unlock(&cpu_hotplug.lock);
> > > + /* Signal the writer is done, no fast path yet. */
> > > + __cpuhp_state = readers_slow;
> > > + wake_up_all(&cpuhp_readers);
> > > +
> > > + /*
> > > + * The wait_event()/wake_up_all() prevents the race where the readers
> > > + * are delayed between fetching __cpuhp_state and blocking.
> > > + */
> > > +
> > > + /* See percpu_up_write(); readers will no longer attempt to block. */
> > > + synchronize_sched();
> >
> > Shouldn't you move wake_up_all(&cpuhp_readers) down after
> > synchronize_sched() (or add another one) ? To ensure that a reader can't
> > see state = BLOCK after wakeup().
>
> Well, if they are blocked, the wake_up_all() will do an actual
> try_to_wake_up() which issues a MB as per smp_mb__before_spinlock().
Yes. Everything is fine with the already blocked readers.
I meant the new reader which still can see state = BLOCK after we
do wakeup(), but I didn't notice it should do __wait_event() which
takes the lock unconditionally, it must see the change after that.
> Right?
Yes, I was wrong, thanks.
Oleg.
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