linux-kernel - Re: [PATCH] include/log2.h: Fix rounddown_pow_of

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Date:	Sat, 19 Nov 2011 10:26:58 +0100
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Andrei Warkentin <andreiw@...are.com>,
	linux-kernel@...r.kernel.org,
	Rolf Eike Beer <eike-kernel@...tec.de>,
	opensuse-kernel@...nsuse.org,
	Sergiu Iordache <sergiu@...omium.org>,
	Eddie Wai <eddie.wai@...adcom.com>,
	Jayamohan Kallickal <jayamohan.kallickal@...lex.com>,
	Guennadi Liakhovetski <g.liakhovetski@....de>
Subject: Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)

Il 18/11/2011 00:05, Andrew Morton ha scritto:
> On Wed, 16 Nov 2011 14:56:06 -0500
> Andrei Warkentin<andreiw@...are.com>  wrote:
>
>> 1 is a power of two, therefore rounddown_pow_of_two(1) should return 1. It does
>> in case the argument is a variable but in case it's a constant it behaves
>> wrong and returns 0. Probably nobody ever did it so this was never noticed,
>> however net/drivers/vmxnet3 with latest GCC does and breaks on unicpu systems.
>>
>> This is similar to Rolf's patch to roundup_pow_of_two(1).
>>
>> Cc: Rolf Eike Beer<eike-kernel@...tec.de>
>> Cc: opensuse-kernel@...nsuse.org
>> Reviewed-by: Jesper Juhl<jj@...osbits.net>
>> Signed-off-by: Andrei Warkentin<andreiw@...are.com>
>> ---
>>   include/linux/log2.h |    2 +-
>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/include/linux/log2.h b/include/linux/log2.h
>> index 25b8086..ccda848 100644
>> --- a/include/linux/log2.h
>> +++ b/include/linux/log2.h
>> @@ -185,7 +185,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
>>   #define rounddown_pow_of_two(n)			\
>>   (						\
>>   	__builtin_constant_p(n) ? (		\
>> -		(n == 1) ? 0 :			\
>> +		(n == 1) ? 1 :			\
>>   		(1UL<<  ilog2(n))) :		\
>>   	__rounddown_pow_of_two(n)		\
>>    )
>
> I assume that nobody has gone off and checked whether all current
> callers will survive this change.  If they had, they'd have looked in
> drivers/char/ramoops.c and seen:
>
> 	rounddown_pow_of_two(pdata->mem_size);
> 	rounddown_pow_of_two(pdata->record_size);
>
> These operations are no-ops.  It should be
>
> 	pdata->mem_size = rounddown_pow_of_two(pdata->mem_size);
> 	pdata->record_size = rounddown_pow_of_two(pdata->record_size);
>
> Marco or Sergio: please fix, test and send it over sometime?

Ok, I'll look at it.

Marco
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