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Message-ID: <4A81331F.7010908@gmail.com>
Date: Tue, 11 Aug 2009 11:00:15 +0200
From: Jiri Slaby <jirislaby@gmail.com>
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To: roel kluin <roel.kluin@gmail.com>
CC: Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] applicom: Prevent unsigned wrap in ac_interrupt()
References: <4A7D961A.1020303@gmail.com> <4A7F31F6.9000706@gmail.com> <25e057c00908110154g48241b85td089e6726109b581@mail.gmail.com>
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On 08/11/2009 10:54 AM, roel kluin wrote:
>>> unsigned i wraps if this occurs in the first iteration.
>>
>> Could you elaborate? I don't quite understand the point.
> 
> `i' is unsigned. The last test in the loop is:
> 
> if(readb(apbs[i].RamIO + RAM_IT_TO_PC)) {
>         /* There's another int waiting on this card */
>         spin_unlock(&apbs[i].mutex);
>         i--;
> } else {
>         spin_unlock(&apbs[i].mutex);
> }
> 
> In the first iteration `i' is 0, so if this condition evaluates to true
> then `i' becomes 0xffffffff (since it's unsigned), the for loop test
> fails and the i++ never occurs.

Hmm, no. This is not how three `for' expressions are evaluated. The CFG
of "for (a; b; c) d;" is "a->(b->d->c)*". Read 6.8.5.3 of ANSI C99.

I.e. 0xff increments back to 0.

Am I still missing something?