linux-kernel - Re: [PATCH] lib: One less subtraction in binary search iterations.

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Date:	Tue, 9 Jul 2013 13:17:45 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Wedson Almeida Filho <wedsonaf@...il.com>
CC:	Joe Perches <joe@...ches.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Tim Abbott" <tabbott@...lice.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib: One less subtraction in binary search iterations.

On 07/09/2013 09:21 AM, Wedson Almeida Filho wrote:
> On Sat, Jul 6, 2013 at 9:59 PM, Joe Perches <joe@...ches.com> wrote:
>>
>> Not correct.
>>
>>>       while (start < end) {
>>> -             size_t mid = start + (end - start) / 2;
>>> +             size_t mid = (start + end) / 2;
>>
>>         size_t start = 0x80000000;
>>         size_t end   = 0x80000001;
> 
> Good point, they aren't equivalent in all cases.
> 
> For the overflow to happen though, we need an array with at least
> N/2+1 entries, where N is the address space size. The array wouldn't
> fit in addressable memory if the element size is greater than 1, so
> this can only really happen when the element size is 1. Even then, it
> would require the kernel range to be greater than half of all
> addressable memory, and allow an allocation taking that much memory. I
> don't know all architectures where linux runs, but I don't think such
> configuration is likely to exist.
> 

It does. In ARC port (arch/arc), the untranslated address space starts at
0x8000_0000 and this is where kernel is linked at. So all ARC kernel addresses
(code/data) lie in that range. This means you don't need special corner case for
this trip on ARC - it will break rightaway - unless I'm missing something.

P.S. Sorry for not replying earlier than ur v2.

-Vineet
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