Date: Thu, 29 Nov 2012 14:34:52 -0500
From: Benjamin LaHaise <bcrl@kvack.org>
To: Kent Overstreet <koverstreet@google.com>
Cc: Andi Kleen <andi@firstfloor.org>, linux-kernel@vger.kernel.org,
        linux-aio@kvack.org, linux-fsdevel@vger.kernel.org, zab@redhat.com,
        jmoyer@redhat.com, axboe@kernel.dk, viro@zeniv.linux.org.uk
Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting
Message-ID: <20121129193452.GI19042@kvack.org>
References: <1354121029-1376-1-git-send-email-koverstreet@google.com> <1354121029-1376-23-git-send-email-koverstreet@google.com> <m28v9kgsen.fsf@firstfloor.org> <20121129185720.GE15094@google.com> <20121129185953.GW16230@one.firstfloor.org> <20121129191214.GG15094@google.com> <20121129192003.GX16230@one.firstfloor.org> <20121129192925.GH15094@google.com>
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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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