linux-kernel - Re: [patch 09/14] fs: use RCU / seqlock logic for reverse and multi-step operaitons

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Date:	Mon, 30 Mar 2009 14:29:02 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 09/14] fs: use RCU / seqlock logic for reverse and multi-step operaitons

On Mon, Mar 30, 2009 at 02:16:49PM +0200, Andi Kleen wrote:
> npiggin@...e.de writes:
> 
> > 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 is the hardest bit.
> 
> General thoughts: is there a way to add a self testing infrastructure
> to this. e.g. by having more sequence counts per object (only enabled
> in the debug case, so it doesn't matter when cache line bounces) and lots of 
> checks? 
> 
> I suppose that would lower the work needed of actually fixing this to 
> work significantly.

Might be a good idea. I'll think about whether it can be done.

Note that I *think* the idea is pretty sound, but I'm just not
quite sure about checking for parent being deleted when we're 
walking back up the tree -- d_unhashed() doesn't seem to work
because we can encounter unhashed parents by design. We might
just need another d_flag...

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