linux-kernel - Re: Found it! (was Re: [3.10] Oopses in kmem_cache_allocate() via prepare

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Message-ID: <20131202164601.GF10323@ZenIV.linux.org.uk>
Date:	Mon, 2 Dec 2013 16:46:01 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Simon Kirby <sim@...tway.ca>,
	Ian Applegate <ia@...udflare.com>,
	Christoph Lameter <cl@...two.org>,
	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...ionio.com>
Subject: Re: Found it! (was Re: [3.10] Oopses in kmem_cache_allocate() via
 prepare_creds())

On Mon, Dec 02, 2013 at 05:27:55PM +0100, Ingo Molnar wrote:

> It's not like there should be many (any?) VFS operations where a pipe 
> is used via i_mutex and pipe->mutex in parallel, which would improve 
> scalability - so I don't see the scalability advantage. (But I might 
> be missing something)
> 
> Barring such kind of workload the extra mutex just adds extra 
> micro-costs because now two locks have to be taken on 
> creation/destruction, plus it adds extra complexity and races.
> 
> So unless I'm missing something obvious, another good fix would be to 
> just revert pipe->mutex and rely on i_mutex as before?

You are missing the extra shitloads of complexity in ->i_mutex ordering,
and ->i_mutex is already used for too many things...
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