linux-kernel - Re: [PATCH] drivers: target: target_core_mod: use div64_u64

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Date:	Wed, 08 Jan 2014 22:40:24 +0800
From:	Chen Gang <gang.chen.5i5j@...il.com>
To:	Hannes Reinecke <hare@...e.de>
CC:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	James Hogan <james.hogan@...tec.com>,
	linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH] drivers: target: target_core_mod: use div64_u64_rem()
 instead of operator '%' for u64

On 01/08/2014 03:32 PM, Hannes Reinecke wrote:
> On 12/24/2013 04:35 AM, Chen Gang wrote:
>> On 12/23/2013 02:51 PM, Nicholas A. Bellinger wrote:
>>> On Sun, 2013-12-22 at 17:17 +0800, Chen Gang wrote:
>>>> On 12/22/2013 10:56 AM, Nicholas A. Bellinger wrote:
>>>>> Hi Chen,
>>>>>
>>>>> On Sat, 2013-12-21 at 10:08 +0800, Chen Gang wrote:
>>>>>> In kernel, need use div64_u64_rem() instead of operator '%' for u64, or
>>>>>> can not pass compiling (with allmodconfig under metag):
>>>>>>
>>>>>>     MODPOST 2909 modules
>>>>>>   ERROR: "__umoddi3" [drivers/target/target_core_mod.ko] undefined!
>>>>>>
>>>>>> Also need u64 type cast for u32 variable multiply u32 variable, or will
>>>>>> cause type overflow issue.
>>>>>>
>>>>>>
>>>>>> Signed-off-by: Chen Gang <gang.chen.5i5j@...il.com>
>>>>>> ---
>>>>>>  drivers/target/target_core_alua.c | 3 ++-
>>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>
>>>>>
>>>>> FYI, this unsigned long long division in core_alua_state_lba_dependent()
>>>>> was fixed for 32-bit in linux-next >= 12192013 code.
>>>>>
>>>>
>>>> OK, thanks.
>>>>
>>>> The related fix patch changed "start_lba = lba % ..." to "start_lba =
>>>> lba / ...", and also assumed "segment_size * segment_mult" is still
>>>> within u32 (can not cause type over flow).
>>>>
>>>> I guess the original author already knew about them, and intended to do
>>>> like that (if not, please let me know, thanks).
>>>>
>>>
>>> Sorry, your correct that the original code is using modulo division to
>>> calculate start_lba.
>>>
>>
>> Oh, that's all right, (in fact, don't need sorry), I am not quite
>> familiar with the details, so need related member help check it.  :-)
>>
>>> Hannes, can you please double check this below..?
>>>
>>
>> Please help check when have time, thanks.
>>
> I would even convert segment_size and segment_mult to u64,
> to ensure no overflows occur:
> 
> diff --git a/drivers/target/target_core_alua.c
> b/drivers/target/target_core_alua
> .c
> index 9b1856d..54b1e52 100644
> --- a/drivers/target/target_core_alua.c
> +++ b/drivers/target/target_core_alua.c
> @@ -477,8 +477,7 @@ static inline int core_alua_state_lba_dependent(
>         u8 *alua_ascq)
>  {
>         struct se_device *dev = cmd->se_dev;
> -       u32 segment_size, segment_mult, sectors;
> -       u64 lba;
> +       u64 segment_size, segment_mult, sectors, lba;
> 
>         /* Only need to check for cdb actually containing LBAs */
>         if (!(cmd->se_cmd_flags & SCF_SCSI_DATA_CDB))
> 
> 

OK, thanks.

> Other than that the sector_div() patch is correct.
> 

So we really need use '/' instead of original '%'?


Thanks
-- 
Chen Gang

Open, share and attitude like air, water and life which God blessed
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