linux-kernel - Re: [PATCH 22/25] Generic dynamic per cpu refcounting

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Message-ID: <20121129193452.GI19042@kvack.org>
Date:	Thu, 29 Nov 2012 14:34:52 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org, zab@...hat.com,
	jmoyer@...hat.com, axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting

On Thu, Nov 29, 2012 at 11:29:25AM -0800, Kent Overstreet wrote:
> There's some kind of symmetry going on here, and if I'd been awake more
> in college I could probably say exactly why it works, but it does.

I think the catch is that using only a 32 bit counter is something the 
user could arbitrarily control the sum of all parts.  I think a 64 bit 
counter may be required to ensure no overflow occurs.  Otherwise, an 
overflow could result in a premature free when there are still 2^32 
objects active thanks to a malicious user (possible on systems with lots 
of memory these days -- remote, but possible).

		-ben
-- 
"Thought is the essence of where you are now."
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