svn commit: r241889 - in user/andre/tcp_workqueue/sys: arm/arm cddl/compat/opensolaris/kern cddl/contrib/opensolaris/uts/common/dtrace cddl/contrib/opensolaris/uts/common/fs/zfs ddb dev/acpica dev/...

[Andre Oppermann]

On 24.10.2012 17:09, Attilio Rao wrote:
> On Wed, Oct 24, 2012 at 3:45 PM, John Baldwin <jhb at freebsd.org> wrote:
>> On Wednesday, October 24, 2012 10:34:34 am Attilio Rao wrote:
>>> On Wed, Oct 24, 2012 at 3:05 PM, John Baldwin <jhb at freebsd.org> wrote:
>>>> On Tuesday, October 23, 2012 7:20:04 pm Andre Oppermann wrote:
>>>>> On 24.10.2012 00:15, mdf at FreeBSD.org wrote:
>>>>>> On Tue, Oct 23, 2012 at 7:41 AM, Andre Oppermann <andre at freebsd.org>
>> wrote:
>>>>>>> Struct mtx and MTX_SYSINIT always occur as pair next to each other.
>>>>>>
>>>>>> That doesn't matter.  Language basics like variable definitions should
>>>>>> not be obscured by macros.  It either takes longer to figure out what
>>>>>> a variable is (because one needs to look up the definition of the
>>>>>> macro) or makes it almost impossible (because now e.g. cscope doesn't
>>>>>> know this is a variable definition.
>>>>>
>>>>> Sigh, cscope doesn't expand macros?
>>>>>
>>>>> Is there a way to do the cache line alignment in a sane way without
>>>>> littering __aligned(CACHE_LINE_SIZE) all over the place?
>>>>
>>>> I was hoping to do something with an anonymous union or some such like:
>>>>
>>>> union mtx_aligned {
>>>>          struct mtx;
>>>>          char[roundup2(sizeof(struct mtx), CACHE_LINE_SIZE)];
>>>> }
>>>>
>>>> I don't know if there is a useful way to define an 'aligned mutex' type
>>>> that will transparently map to a 'struct mtx', e.g.:
>>>>
>>>> typedef struct mtx __aligned(CACHE_LINE_SIZE) aligned_mtx_t;
>>>
>>> Unfortunately that doesn't work as I've verified with alc@ few months ago.
>>> The __aligned() attribute only works with structures definition, not
>>> objects declaration.
>>
>> Are you saying that the typedef doesn't (I expect it doesn't), or that this
>> doesn't:
>>
>> struct mtx foo __aligned(CACHE_LINE_SIZE);
>
> I meant to say that such notation won't address the padding issue
> which is as import as the alignment. Infact, for sensitive locks,
> having just an aligned object is not really useful if the cacheline
> gets shared.

As far as I understand __aligned() not only aligns the start of the
object but also ensures that is padded on a multiple of the alignment
after the object.  So explicit padding after it is not necessary.

> In the end you will need to use explicit padding or use __aligned in
> the struct definition, which cannot be used as a general pattern.

-- 
Andre