Messages in this thread | | | Date | Wed, 8 Jun 2011 14:42:54 -0400 | From | Vivek Goyal <> | 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?
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
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