lkml.org 
[lkml]   [2011]   [Jun]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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
>


\
 
 \ /
  Last update: 2011-06-08 20:39    [W:0.077 / U:0.096 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site