Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Peter Zijlstra]

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().

The woken task will get a MB from passing through the context switch to
make it actually run. And therefore; like Paul's comment says; it cannot
observe the previous BLOCK state but must indeed see the just issued
SLOW state.

Right?
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