Re: [patch 11/33] fs: dcache scale subdirs

[Nick Piggin]

On Mon, Jun 21, 2010 at 11:02:37PM -0700, John Stultz wrote:
> On Mon, 2010-06-21 at 16:55 +0200, Peter Zijlstra wrote:
> > On Tue, 2010-06-22 at 00:48 +1000, Nick Piggin wrote:
> > > > Right, so I was staring at the -rt splat, so its John who created that
> > > > wreckage?
> > > 
> > > It was, but apparently they saw an RCU bug there somewhere and hit it
> > > with the big hammer. I haven't been able to reproduce it on a non-rt
> > > kernel yet, and I see yet why RCU is not good enough here.
> > 
> > John, could you describe the failure you spotted?
> 
> The problem was that the rcu_read_lock() on the dentry ascending wasn't
> preventing d_put/d_kill from removing entries from the parent node. So
> the next entry we tried to follow was invalid. So we were getting odd
> oopses from select_parent().

Oh, ah OK that makes sense. I was thinking it was a use after grace
period problem. Hmm, I'll think about whether we can fix it better.

> 
> I'm not as familiar with the rcu rules there, so the patch I made just
> held the locks as it went down the chain. Not ideal of course, but still
> an improvement over the dcache_lock that was there prior.
> 
> Peter: I'm sorry, I've been out for a few days. Can you give me some
> background on what brought this up and what -rt splat you mean?
> 
> thanks
> -john
> 
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