linux-kernel - Re: [rfc][patch] store-free path walking

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Date:	Wed, 7 Oct 2009 07:56:33 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [rfc][patch] store-free path walking

On Wed, 7 Oct 2009, Nick Piggin wrote:
> 
> OK, I have a really basic patch that does store-free path walking
> (except on the final element).

Yay!

> dbench is pretty nasty still because it seems to do a lot of stupid 
> things like reading from /proc/mounts all the time.

You should largely forget about dbench, it can certainly be a useful 
benchmark, but at the same time it's certainly not a _meaningful_ one.
There are better things to try.

> The seqlock in the dentry is for getting consistent name,len pointer,
> and also not giving a false positive if a rename has partially
> overwritten the name string (false negatives are always fine in the
> lock free lookup path but false positives are not). Possibly we
> could make do with a per-sb seqlock for this (or just rename_lock).

My plan was always to just use rename_lock, and only do it at the outer 
level (and do it for both lookup failures _and_ for the success case). 
Your approach is _way_ more conservative than I would have done, and also 
potentially much slower due to the seqlock-per-path-component thing. 

Remember: seqlocks are almost free on x86, but they can be reasonably 
expensive in other places.

Hmm. Regardless, this very much does look like what I envisioned, apart 
from details like that. And maybe your per-dentry seqlock is the right 
choice. On x86, it certainly doesn't have the performance issues it could 
have in other places.

I'd like Al to take a look, if he's around.

			Linus
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