linux-kernel - Re: [PATCH 1/2] IB/hfi1: Fix a parameter of find_first

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Date:   Fri, 26 Aug 2016 15:34:48 -0400
From:   Doug Ledford <dledford@...hat.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        mike.marciniszyn@...el.com, dennis.dalessandro@...el.com,
        sean.hefty@...el.com, hal.rosenstock@...il.com,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 1/2] IB/hfi1: Fix a parameter of find_first_bit.

On 8/26/2016 3:29 PM, Leon Romanovsky wrote:
> On Fri, Aug 26, 2016 at 02:01:55PM -0400, Doug Ledford wrote:
>> On 8/26/2016 9:35 AM, Doug Ledford wrote:
>>> On 8/26/2016 12:49 AM, Christophe JAILLET wrote:
>>>> The 2nd parameter of 'find_first_bit' is the number of bits to search.
>>>> In this case, we are passing 'sizeof(unsigned long)' which is likely to
>>>> be 4 or 8.
>>>
>>> If the size can be 4 or 8, then using 64 universally is not correct.
>>> Why not use sizeof() * 8 (or << 3)?
>>
>> Better yet, why not put this patch in the kernel first:
>>
>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>> index d96a6118d26a..a8838c87668e 100644
>> --- a/include/linux/kernel.h
>> +++ b/include/linux/kernel.h
>> @@ -52,6 +52,8 @@
>>
>>  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) +
>> __must_be_array(arr))
>>
>> +#define bitsizeof(x)   (sizeof((x)) << 3)
>> +
>>  #define u64_to_user_ptr(x) (           \
>>  {                                      \
>>         typecheck(u64, x);              \
>>
>> then start going around replacing all these hard coded numbers with the
>> use of bitsizeof().  It can be applied not just to the find_first*bit()
>> routines, but to a bunch of other routines too.  Just look at
>> include/linux/bitmap.h and any that have nbits as an argument are
>> candidates.
> 
> There is BITS_PER_LONG define for that. There is actual use of it in mlx5 for
> the similar code pieces.

BITS_PER_LONG only works if your bitfield is a single long.  It doesn't
work for other bitfields.  What I posted above will work for anything.


-- 
Doug Ledford <dledford@...hat.com>
    GPG Key ID: 0E572FDD



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