Re: [dpdk-dev] lib: include rte_memory.h for __rte_cache_aligned

[Jia Yu <jyu@vmware.com>]

Hi Neil,

Moving __rte_cache_aligned right after struct keyword will help. On the
other hand, enforcing this rule for existing (100+) and future definitions
will be difficult. It¹s clearer and a good practice to include header file
explicitly.

Thanks,
Jia


On 12/9/14, 7:22 AM, "Neil Horman" <nhorman@tuxdriver.com> wrote:

>On Tue, Dec 09, 2014 at 09:53:18AM +0100, Olivier MATZ wrote:
>> Hi Neil,
>> 
>> On 12/08/2014 04:04 PM, Neil Horman wrote:
>> >On Fri, Nov 07, 2014 at 09:28:09AM -0800, Jia Yu wrote:
>> >>Include rte_memory.h for lib files that use __rte_cache_aligned
>> >>attribute.
>> >>
>> >>Signed-off-by: Jia Yu <jyu@vmware.com>
>> >>
>> >Why?  I presume there was a build break or something.  Please repost
>>with a
>> >changelog that details what this patch is for.
>> >Neil
>> 
>> I don't know if Yu's issue was the same, but I had a very "fun" issue
>> with __rte_cache_aligned in my application. Consider the following code:
>> 
>> 	struct per_core_foo {
>> 		...
>> 	} __rte_cache_aligned;
>> 
>> 	struct global_foo {
>> 		struct per_core_foo foo[RTE_MAX_CORE];
>> 	};
>> 
>> If __rte_cache_aligned is not defined (rte_memory.h is not included),
>> the code compiles but the structure is not aligned... it defines the
>> structure and creates a global variable called __rte_cache_aligned.
>> And this can lead to really bad things if this code is in a .h that
>> is included by files that may or may not include rte_memory.h
>> 
>> I have no idea about how we could prevent this issue, except using
>> __attribute__((aligned(CACHE_LINE))) instead of __rte_cache_aligned.
>> 
>> Anyway this could probably explain the willing to include rte_memory.h
>> everywhere.
>> 
>> Regards,
>> Olivier
>> 
>> 
>
>So, that is a great explination, and would be good to have in the
>changelog.
>
>Also, to avoid the problem that you describe, while its preferred to have
>it at
>the end of a struct, you can also put the alignment attribute right after
>the
>struct keyword in gcc:
>https://urldefense.proofpoint.com/v2/url?u=https-3A__gcc.gnu.org_onlinedoc
>s_gcc_Attribute-2DSyntax.html-23Attribute-2DSyntax&d=AAIBAg&c=Sqcl0Ez6M0X8
>aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=q34pQj5yiMxs5OseYCxXLQ&m=mIyHF3ASZxRmbPs
>acyMyIABQlSafUdV9PqknKAtfOuI&s=pKoAAkIYRX31K-gR5oSwpcA5mLj4nG7uEzyh0z_uwxU
>&e= 
>
>That seems like it would solve the problem going forward.
>
>Neil
>