Messages in this thread | | | Subject | Re: [PATCH 5/5] CFQ: use proper locking for cache of last hit cic | From | Paul Bolle <> | Date | Wed, 08 Jun 2011 21:32:52 +0200 |
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On Wed, 2011-06-08 at 14:42 -0400, Vivek Goyal wrote: > 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.
That could be because I don't really get all the RCU voodoo, nor how this all interacts with the io_contect->lock here, so I probably asked an impossible question.
> 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?
Jens updated the code to if (rcu_dereference(ioc->ioc_data) == cic) { spin_lock(&ioc->lock); rcu_assign_pointer(ioc->ioc_data, NULL); spin_unlock(&ioc->lock); }
I basically suggested (except for an apparently useless rcu_read_lock() / rcu_read_unlock() pair and using spin_lock_irqsave() etc.): spin_lock(&ioc->lock); if (rcu_dereference(ioc->ioc_data) == cic) rcu_assign_pointer(ioc->ioc_data, NULL); spin_unlock(&ioc->lock);
Ie, I thought that reads of and updates to ioc->ioc_data should be done with ioc->lock held. My reasoning was that in Jens code ioc->ioc_data might already be updated (by another thread, or whatever) and thus not be equal to cic by the time it's updated to NULL. See, in my understanding ioc->ioc_data could be equal to cic when it's rcu_dereference()'d, but unequal to cic by the time it's rcu_assign_pointer()'d to NULL.
Paul Bolle
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