Skip site navigation (1)Skip section navigation (2)
Date:      Mon, 4 Jun 2012 17:27:49 +0300
From:      Konstantin Belousov <kostikbel@gmail.com>
To:        Attilio Rao <attilio@freebsd.org>
Cc:        svn-src-head@freebsd.org, Tijl Coosemans <tijl@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org
Subject:   Re: svn commit: r236456 - in head/sys: amd64/include i386/include
Message-ID:  <20120604142749.GB85127@deviant.kiev.zoral.com.ua>
In-Reply-To: <CAJ-FndDENiKT5iG0gCEbxdC45yOXEgH7P_=72SXPRdFRxB_DKw@mail.gmail.com>
References:  <201206021810.q52IAGZA004238@svn.freebsd.org> <4FCC873B.90104@freebsd.org> <20120604125050.GA85127@deviant.kiev.zoral.com.ua> <CAJ-FndDENiKT5iG0gCEbxdC45yOXEgH7P_=72SXPRdFRxB_DKw@mail.gmail.com>

next in thread | previous in thread | raw e-mail | index | archive | help

--OwLcNYc0lM97+oe1
Content-Type: text/plain; charset=koi8-r
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Mon, Jun 04, 2012 at 02:58:57PM +0100, Attilio Rao wrote:
> 2012/6/4 Konstantin Belousov <kostikbel@gmail.com>:
> > On Mon, Jun 04, 2012 at 12:00:27PM +0200, Tijl Coosemans wrote:
> >> On 02-06-2012 20:10, Konstantin Belousov wrote:
> >> > Author: kib
> >> > Date: Sat Jun =9A2 18:10:16 2012
> >> > New Revision: 236456
> >> > URL: http://svn.freebsd.org/changeset/base/236456
> >> >
> >> > Log:
> >> > =9A Use plain store for atomic_store_rel on x86, instead of implicit=
ly
> >> > =9A locked xchg instruction. =9AIA32 memory model guarantees that st=
ore has
> >> > =9A release semantic, since stores cannot pass loads or stores.
> >>
> >> They can pass non-temporal stores can't they?
> > Sure. But (our) barriers only work for WB memory accesses, in respect t=
o other
> > WB memory accesses.
> >
> > The atomic(9) contains not quite explicit mention of the requirement,
> > for ia32 and more direct notion for ia64. It could probably be reworded=
 to
> > mention memory access type explicitely for ia32 too.
>=20
> I don't think this is right.
> What if I want to use NTI in a block of code locked? What if I want to
> use CLFLUSH? I simply cannot do that now because of the reordering
> requirement.
Assuming that NTI means "Non Temporal Instruction", Intel explicit
requirement is to use fence barrier if order shall be ensured. This,
as well as CLFLUSH use, is somewhat commonly documented. More, CLFLUSH
is documented by Intel to _not_ serialize with any other fencing or
serialization instruction, except MFENCE. So xchg-based _store_rel is
not different from mov-based _store_rel for CLFLUSH and non-temporal
ops.

I do not see how you note is relevant.

> Also, there is the more worrisome case of the string operations. If
> gcc/clang optimize the code in order to do string operations between
> locked path, this is not valid anymore as they can be reordered
> against the _rel() barrier.
They cannot. Fast string operation volatile store order only among
string operation itself, the operation cannot pass sequential store.
The store used in _store_rel thus cannot be passed by fast string
optimizations.

I do not see how you note is relevant there, again.

>=20
> However, we should consider atomic(9) as a script for MI requirement
> of our locking primitives among the architectures. Right now too many
> things live on assumptions of people doing patches (like this case)
> rather than actually working on a common policy of what we can easilly
> support and what we can't.
>=20
> I also wondered often if we should use *fence on architectures
> supporting them, by default, because of the possibility to use FPU now
> (which wasn't present back in the day) and thus we cannot really
> guarantee memory ordering over stores of memory areas bigger than a
> quad-word. If we don't want to add the burden, we should explicitely
> mention that in atomic(9) or any other place.
The proposal to use fence explicitely contradicts recommendations from
the AMD Optimization Guide, which, JFYI, I cited in the updated comment
in the patch.

How is FPU relevant to the memory model discussion, I left out of the
answer.

>=20
> Definitively: I think this patch violates some edge cases. Please back it=
 out.
No. I explicitely inform you that I consider the backout request
as frivolous, technically unfounded, and that I will not back it out.



--OwLcNYc0lM97+oe1
Content-Type: application/pgp-signature
Content-Disposition: inline

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.12 (FreeBSD)

iEYEARECAAYFAk/MxeUACgkQC3+MBN1Mb4hpagCfSnSKT0ZdhVH3/wUvvdBe5TCH
CeQAmwY/wASzpG9nu7uOB9GbxPywH9pb
=GTZq
-----END PGP SIGNATURE-----

--OwLcNYc0lM97+oe1--



Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20120604142749.GB85127>