linux-kernel - Re: [patch 16/52] fs: dcache RCU for multi-step operaitons

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Date:	Fri, 25 Jun 2010 01:03:34 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>
Subject: Re: [patch 16/52] fs: dcache RCU for multi-step operaitons

On Thu, Jun 24, 2010 at 09:58:10AM +0200, Peter Zijlstra wrote:
> On Thu, 2010-06-24 at 13:02 +1000, npiggin@...e.de wrote:
> > plain text document attachment (fs-dcache_lock-multi-step.patch)
> > The remaining usages for dcache_lock is to allow atomic, multi-step read-side
> > operations over the directory tree by excluding modifications to the tree.
> > Also, to walk in the leaf->root direction in the tree where we don't have
> > a natural d_lock ordering.
> > 
> > This could be accomplished by taking every d_lock, but this would mean a
> > huge number of locks and actually gets very tricky.
> > 
> > Solve this instead by using the rename seqlock for multi-step read-side
> > operations. Insert operations are not serialised. Delete operations are
> > tricky when walking up the directory our parent might have been deleted
> > when dropping locks so also need to check and retry for that.
> > 
> > XXX: hmm, we could of course just take the rename lock if there is any worry
> > about livelock. Most of these are slow paths.
> 
> 
> Ah, does this address John's issue? 

This is where John's issue is introduced. I actually again couldn't
see the problem (thought I saw a problem, then lost it!).

Got to think about it and test more... I couldn't reproduce the problem
mind you, but I was testing mainline wheras bug was seen on -rt.
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