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

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Message-ID: <1277186557.1791.7.camel@work-vm>
Date:	Mon, 21 Jun 2010 23:02:37 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Nick Piggin <npiggin@...e.de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, John Kacur <jkacur@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 11/33] fs: dcache scale subdirs

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().

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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