Re: [PATCH 5/5] CFQ: use proper locking for cache of last hit cic

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jun 08, 2011 at 08:18:44PM +0200, Paul Bolle wrote:
> On Mon, 2011-06-06 at 05:06 +0200, Jens Axboe wrote:
> > On 2011-06-05 18:26, Paul Bolle wrote:
> > > @@ -2704,8 +2706,13 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
> > >  	smp_wmb();
> > >  	cic->key = cfqd_dead_key(cfqd);
> > >  
> > > -	if (ioc->last_cic == cic)
> > > +	spin_lock_irqsave(&ioc->lock, flags);
> > > +	rcu_read_lock();
> > > +	last_cic = rcu_dereference(ioc->last_cic);
> > > +	rcu_read_unlock();
> > > +	if (last_cic == cic)
> > >  		rcu_assign_pointer(ioc->last_cic, NULL);
> > > +	spin_unlock_irqrestore(&ioc->lock, flags);
> > 
> > We don't need the ioc->lock for checking the cache, it would in fact
> > defeat the purpose of using RCU.
> 
> Just to show that I'm RCU-challenged, is that because:
> 1) my use of locking on ioc->lock defends for a race that is not
> actually possible; or
> 2) the worst thing that could happen is that some new and correct value
> of ioc->last_cic will be replaced with NULL, which is simply not a big
> deal?

I don't understand this point. All ioc->ioc_data updates are under
ioc->lock except the one __cfq_exit_single_io_context() and that's what
jens patch fixed. So clearly there was atleast one race where we were
doing a value update without taking appropriate lock. 

Why do you think that some new and correct value will be replaced
by NULL?

Thanks
Vivek