Messages in this thread | | | Date | Wed, 8 Jun 2011 11:37:19 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 5/5] CFQ: use proper locking for cache of last hit cic |
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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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