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

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Date:	Wed, 8 Jun 2011 11:37:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Paul Bolle <pebolle@...cali.nl>
Cc:	Jens Axboe <axboe@...nel.dk>, Vivek Goyal <vgoyal@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] CFQ: use proper locking for cache of last hit cic

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?

My likely incorrect guess is that acquiring the lock excludes any
updates, so that there is no point in the RCU read-side critical
section.  But I don't claim to understand this code.

> > But this hunk will clash with the
> > merged part anyway.
> 
> Looking at Paul's feedback I do have a feeling that in your commit
> (9b50902db5eb8a220160fb89e95aa11967998d12, "cfq-iosched: fix locking
> around ioc->ioc_data assignment") the line:
> 	if (rcu_dereference(ioc->ioc_data) == cic) {
> 
> could actually be replaced with:
> 	if (rcu_access_pointer(ioc->ioc_data) == cic) {
> 
> Is that correct?

If you are not actually dereferencing the pointer, then yes, you
can use rcu_access_pointer() instead of rcu_dereference().  In
this case, the pointer is being compared against, not dereferenced,
so rcu_access_pointer() should do it.

							Thanx, Paul

> > [...]
> > See Pauls comment on this part.
> 
> You seem to be offline right now. When you're back online, could you
> please say whether or not you accept the two renaming patches that you
> have rejected a few days ago (and for which I gave some follow up
> arguments)? After that I'll send an update to this patch (and its commit
> message) to reflect Paul's review and your review.
> 
> 
> Paul Bolle
> 
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