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

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Message-ID: <Pine.LNX.4.64.0812101412130.27886@vixen.sonytel.be>
Date:	Wed, 10 Dec 2008 14:15:15 +0100 (CET)
From:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
To:	David Howells <dhowells@...hat.com>
cc:	Yuri Tikhonov <yur@...raft.com>, Wolfgang Denk <wd@...x.de>,
	Detlev Zundel <dzu@...x.de>, linux-kernel@...r.kernel.org,
	Milton Miller <miltonm@....com>, linuxppc-dev@...abs.org,
	Al Viro <viro@...IV.linux.org.uk>,
	Ilya Yanok <yanok@...raft.com>
Subject: Re: Re[2]: [PATCH] fork_init: fix division by zero

On Wed, 10 Dec 2008, David Howells wrote:
> Yuri Tikhonov <yur@...raft.com> wrote:
> >  In any case, adding braces as follows probably would be better:
> > 
> > +     max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
> 
> I think you mean brackets, not braces '{}'.
> 
> >  Right ?
> 
> Definitely not.
> 
> I added this function to the above:
> 
> 	unsigned long alt(void)
> 	{
> 		unsigned long max_threads;
> 		max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
> 		return max_threads;
> 	}
> 
> and ran it through "gcc -S -O2" for x86_64:
> 
> 	jump:
> 		movq    mempages(%rip), %rax
> 		salq    $12, %rax
> 		shrq    $16, %rax
> 		ret
> 	alt:
> 		xorl    %eax, %eax
> 		ret
> 
> Note the difference?  In jump(), x86_64 first multiplies mempages by 4096, and
> _then_ divides by 8*8192.
> 
> In alt(), it just returns 0 because the compiler realised that you're
> multiplying by 0.

The case were the multiplier is 0 (actually smaller than 1, but not integer)
is handled by

	#if (8 * THREAD_SIZE) > PAGE_SIZE
	      max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
	#else
	     ...

> If you're going to bracket the expression, it must be:
> 
> 		max_threads = (mempages * PAGE_SIZE) / (8 * THREAD_SIZE);
> 
> which should be superfluous.

No, `mempages * PAGE_SIZE' may overflow.

With kind regards,

Geert Uytterhoeven
Software Architect

Sony Techsoft Centre Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium

Phone:    +32 (0)2 700 8453
Fax:      +32 (0)2 700 8622
E-mail:   Geert.Uytterhoeven@...ycom.com
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