linux-kernel - Re[2]: [PATCH] fork_init: fix division by zero

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Message-ID: <527599002.20081210205651@emcraft.com>
Date:	Wed, 10 Dec 2008 20:56:51 +0300
From:	Yuri Tikhonov <yur@...raft.com>
To:	Scott Wood <scottwood@...escale.com>
CC:	Al Viro <viro@...IV.linux.org.uk>, Wolfgang Denk <wd@...x.de>,
	Detlev Zundel <dzu@...x.de>, <linux-kernel@...r.kernel.org>,
	Milton Miller <miltonm@....com>, <linuxppc-dev@...abs.org>,
	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>,
	Ilya Yanok <yanok@...raft.com>
Subject: Re[2]: [PATCH] fork_init: fix division by zero


Hello Scott,

On Wednesday, December 10, 2008 you wrote:

> On Wed, Dec 10, 2008 at 01:29:12PM +0300, Yuri Tikhonov wrote:
>> On Wednesday, December 10, 2008 you wrote:
>> > x * y / z is parsed as (x * y) / z, not x * (y / z).
>> 
>>  Here we believe in preprocessor: since all PAGE_SIZE, 8, and 
>> THREAD_SIZE are the constants we expect it will calculate this.
>> 
>>  E.g. here is the result from this line as produced by cross-gcc 
>> 4.2.2:
>> 
>>         lis     r9,0
>>         rlwinm  r29,r29,2,16,29
>>         stw     r29,0(r9)
>> 
>>  As you see - only rotate-left, i.e. multiplication to the constant.

> Yes, and also note that it is masking the result by 0xfffc, to preserve
> the effect of any overflow in (x * y).

 Right, such a mask was the result of missed brackets. Now (see 
"[PATCH][v2] fork_init: fix division by zero" I've just posted) the 
situation is better:

        lis     r9,0
        rlwinm  r29,r29,2,0,29
        stw     r29,0(r9)

 I.e. the mask is 0xFFFFFFFC.

 Regards, Yuri

 --
 Yuri Tikhonov, Senior Software Engineer
 Emcraft Systems, www.emcraft.com

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