Date: Thu, 9 Apr 2015 07:06:03 -0600 From: Warner Losh <imp@bsdimp.com> To: Mateusz Guzik <mjguzik@gmail.com> Cc: Attilio Rao <attilio@FreeBSD.org>, adrian@freebsd.org, Alan Cox <alc@rice.edu>, Konstantin Belousov <kib@FreeBSD.org>, freebsd-arch@freebsd.org Subject: Re: atomic ops Message-ID: <A3DFECF4-ABA7-450C-AC13-2A2672BCF6A8@bsdimp.com> In-Reply-To: <20150409061448.GB6086@dft-labs.eu> References: <20141028025222.GA19223@dft-labs.eu> <20150409061448.GB6086@dft-labs.eu>
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--Apple-Mail=_F8FEC27C-BD9E-4735-8904-E51A12661A9D Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 > On Apr 9, 2015, at 12:14 AM, Mateusz Guzik <mjguzik@gmail.com> wrote: >=20 > On Tue, Oct 28, 2014 at 03:52:22AM +0100, Mateusz Guzik wrote: > [scratching old content so that I hopefully re-state it nicer] >=20 > I would like to reviwe the discussion about memory barriers provided = in > the kernel. >=20 > The kernel (at least on amd64) lacks lightweight barriers providing = only > following guarantees: > - all writes are completed prior to given point > - all reads are completed prior to given point What does completed mean? That=E2=80=99s a very crappy definition since = it means a range of things. (1) The CPU has pushed the writes to cache, but the cache is coherent = across the whole CPU complex. (2) The cache has flushed it to the memory controller / PCIe bridge (3) The memory has actually been updated / The PCIe bridge has pushed = the write to the card. (4) The card has completed its transaction. Which one is it? What=E2=80=99s its purpose? The efficient = implementation requires the definition be towards the start of the list. Convenient programming = of certain devices, however, requires the end=E2=80=A6 Warner --Apple-Mail=_F8FEC27C-BD9E-4735-8904-E51A12661A9D Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP using GPGMail -----BEGIN PGP SIGNATURE----- Comment: GPGTools - https://gpgtools.org iQIcBAEBCgAGBQJVJnk7AAoJEGwc0Sh9sBEA7NkP/jUBhUi64iqzieCh1IgCrg/l Dpecq/QEYmC2XtOVm9YVyoqLQptrxb2zg7LjKoAcjMA8Io9EpxZucsZYohaZnos4 3jAEklukZpxZZ71ulhWBYrmvccn6atIPyzeoSWPWS+okTvDXBRYZ4OYAlPKvTMso PLM9aSAvQ6TxSzsR0LOjYAWax9aM3MHxoqWjgS5poemAkSu+qA+gMTaW0bM/7TEF FgGrEuIrjEwSF2hs2THfytQj/2otv8wPLVALlzgd9W9+DbnqCC+mKMSiXr9yEHCv qeuz5kmEx6QRJs7wb3OYn/st4JTUoIoLu7kNHYGlqYqNCBKzl3GxL2HH58uPasv1 j/FHRNwg2+HVDF/7I5X2NzXwoa9MXeL5WCgZQj6We9DL+qPXutvofzF6MoCLTjmy 9Hr884IXG6APvozopvEp0Xm1FI+UEVeiXSM22SDeqaPH/d/4/A5V3nxdpPzDEjQP uDyrrRW7AWK/h3Ad1UBFVebclnaUILqIR9nzLV8CNXl4J5R4CFM7ae0QEq0H4O7c z8TfyHyQnTaM7XY0FtZ9TOWY1UlzgmPordZtA5EwUvyYz/zrfR300D4lgOLatY51 rYJkwlSEEWJlbxDQnLuw+qFYyt0Kx+2TpifQ9EouiCPnE8hpb6tsVq9YjA7V1jlQ L8wGp9mQINM/E7BBu+cd =IJLJ -----END PGP SIGNATURE----- --Apple-Mail=_F8FEC27C-BD9E-4735-8904-E51A12661A9D--
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