Date: Wed, 24 Oct 2012 16:24:22 +0100 From: Attilio Rao <attilio@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: mdf@freebsd.org, src-committers@freebsd.org, Andre Oppermann <andre@freebsd.org>, Bruce Evans <brde@optusnet.com.au>, svn-src-user@freebsd.org Subject: Re: 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/... Message-ID: <CAJ-FndCDeB7w30PgSwOpMbWT6e%2BR7iQHfa8PU8Pn0NLagCiJxA@mail.gmail.com> In-Reply-To: <CAJ-FndC=zV%2BHN1wr_CnSEY93VHT--w9cYPMhH8P53y%2BLvBSO7g@mail.gmail.com> References: <201210221418.q9MEINkr026751@svn.freebsd.org> <201210241005.38977.jhb@freebsd.org> <CAJ-FndBENEuyaH%2B2Q%2Bigj39tdGmsHh=3arL-Cb2GP3i9WSr_hQ@mail.gmail.com> <201210241045.39211.jhb@freebsd.org> <CAJ-FndC=zV%2BHN1wr_CnSEY93VHT--w9cYPMhH8P53y%2BLvBSO7g@mail.gmail.com>
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On Wed, Oct 24, 2012 at 4:09 PM, Attilio Rao <attilio@freebsd.org> wrote:
> On Wed, Oct 24, 2012 at 3:45 PM, John Baldwin <jhb@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@freebsd.org> wrote:
>>> > On Tuesday, October 23, 2012 7:20:04 pm Andre Oppermann wrote:
>>> >> On 24.10.2012 00:15, mdf@FreeBSD.org wrote:
>>> >> > On Tue, Oct 23, 2012 at 7:41 AM, Andre Oppermann <andre@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.
> 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.
The quickest way I see this can be made general is to have a specific
struct defined in sys/_mutex.h like that
struct mtx_unshare {
struct mtx lock;
char _pad[CACHE_LINE_SIZE - sizeof(struct mtx)];
} __aligned(CACHE_LINE_SIZE);
then let mtx_* functions to accept void ptrs and cast them to struct
mtx as long as the functions enter.
I think that almost all the static/non-member-of-a-struct mutex should
be converted to be struct mtx_unshare, or possibly the most
coarse-grained ones.
Attilio
--
Peace can only be achieved by understanding - A. Einstein
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