From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757383AbbIUXh2 (ORCPT ); Mon, 21 Sep 2015 19:37:28 -0400 Received: from mail-pa0-f44.google.com ([209.85.220.44]:35085 "EHLO mail-pa0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755072AbbIUXh1 (ORCPT ); Mon, 21 Sep 2015 19:37:27 -0400 Date: Tue, 22 Sep 2015 07:37:04 +0800 From: Boqun Feng To: Will Deacon Cc: "linux-kernel@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , Peter Zijlstra , Ingo Molnar , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Thomas Gleixner , "Paul E. McKenney" , Waiman Long Subject: Re: [RFC v2 3/7] powerpc: atomic: Implement atomic{,64}_{add,sub}_return_* variants Message-ID: <20150921233704.GD970@fixme-laptop.cn.ibm.com> References: <1442418575-12297-1-git-send-email-boqun.feng@gmail.com> <1442418575-12297-4-git-send-email-boqun.feng@gmail.com> <20150918165902.GF12837@arm.com> <20150919153310.GB20458@fixme-laptop.cn.ibm.com> <20150920082303.GA1166@fixme-laptop.cn.ibm.com> <20150921222427.GG7356@arm.com> <20150921232656.GC970@fixme-laptop.cn.ibm.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="ieNMXl1Fr3cevapt" Content-Disposition: inline In-Reply-To: <20150921232656.GC970@fixme-laptop.cn.ibm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --ieNMXl1Fr3cevapt Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Sep 22, 2015 at 07:26:56AM +0800, Boqun Feng wrote: > On Mon, Sep 21, 2015 at 11:24:27PM +0100, Will Deacon wrote: > > Hi Boqun, > >=20 > > On Sun, Sep 20, 2015 at 09:23:03AM +0100, Boqun Feng wrote: > > > On Sat, Sep 19, 2015 at 11:33:10PM +0800, Boqun Feng wrote: > > > > On Fri, Sep 18, 2015 at 05:59:02PM +0100, Will Deacon wrote: > > > > > On Wed, Sep 16, 2015 at 04:49:31PM +0100, Boqun Feng wrote: > > > > > > On powerpc, we don't need a general memory barrier to achieve a= cquire and > > > > > > release semantics, so __atomic_op_{acquire,release} can be impl= emented > > > > > > using "lwsync" and "isync". > > > > >=20 > > > > > I'm assuming isync+ctrl isn't transitive, so we need to get to th= e bottom > > > >=20 > > > > Actually the transitivity is still guaranteed here, I think ;-) > >=20 > > The litmus test I'm thinking of is: > >=20 > >=20 > > { > > 0:r2=3Dx; > > 1:r2=3Dx; 1:r5=3Dz; > > 2:r2=3Dz; 2:r4=3Dx; > > } > > P0 | P1 | P2 ; > > li r1,1 | lwz r1,0(r2) | lwz r1,0(r2) ; > > stw r1,0(r2) | cmpw r1,r1 | cmpw r1,r1 ; > > | beq LC00 | beq LC01 ; > > | LC00: | LC01: ; > > | isync | isync ; > > | li r4,1 | lwz r3,0(r4) ; > > | stw r4,0(r5) | ; > > exists > > (1:r1=3D1 /\ 2:r1=3D1 /\ 2:r3=3D0) > >=20 > >=20 > > Which appears to be allowed. I don't think you need to worry about back= wards > > branches for the ctrl+isync construction (none of the current example d= o, > > afaict). > >=20 >=20 > Yes.. my care of backwards branches is not quite related to the topic, I > concerned that mostly because my test is using atomic operation, and I > just want to test the exact asm code. >=20 > > Anyway, all the problematic cases seem to arise when we start mixing > > ACQUIRE/RELEASE accesses with relaxed accesses (i.e. where an access fr= om > > one group reads from an access in the other group). It would be simplest > > to say that this doesn't provide any transitivity guarantees, and that > > an ACQUIRE must always read from a RELEASE if transitivity is required. > >=20 >=20 > Agreed. RELEASE alone doesn't provide transitivity and transitivity is ^^^^^^^ This should be ACQUIRE... > guaranteed only if an ACQUIRE read from a RELEASE. That's exactly the > direction which the link (https://lkml.org/lkml/2015/9/15/836) is > heading to. So I think we are fine here to use ctrl+isync here, right? >=20 > Regards, > Boqun --ieNMXl1Fr3cevapt Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAABCAAGBQJWAJSbAAoJEEl56MO1B/q4g1AH/jvEg1MYW6HucXqK4LQi3mkH s9M1PfEke1+1k8kx6LMxyVvkW/Hd/+YVvJ9ZzfwYb2b84N7vOLRz/f93tJmBsFRA gAhM8Q4MH/H2jWevhho7I4rbZq6H28zTJ3dKDvvg8M1jc7xp91lCwjysPNvgihep tzs86Q+zWO9nk9ho3ETmaKjbDE6fnlFChvwUTG3Ilu71JSk+VpfqeDtjzwrHWcEN 2jOg0Wq/62rIwDeTLp380D+DyFXXKHQYXCfaZfe2EYTLheCrV1tcepRQ+G974RGQ ITB9xAWEmHOUu/f6Uv0WLFLOSkRiROSBifW/RHtKb3ZEhxnHhA5DGBFNRMTLGhI= =JCGX -----END PGP SIGNATURE----- --ieNMXl1Fr3cevapt--