From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932325AbbIZCS5 (ORCPT ); Fri, 25 Sep 2015 22:18:57 -0400 Received: from mail-ob0-f174.google.com ([209.85.214.174]:33397 "EHLO mail-ob0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755180AbbIZCSz (ORCPT ); Fri, 25 Sep 2015 22:18:55 -0400 Date: Sat, 26 Sep 2015 10:18:29 +0800 From: Boqun Feng To: "Paul E. McKenney" Cc: Will Deacon , "linux-kernel@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , Peter Zijlstra , Ingo Molnar , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Thomas Gleixner , Waiman Long Subject: Re: [RFC v2 3/7] powerpc: atomic: Implement atomic{,64}_{add,sub}_return_* variants Message-ID: <20150926021829.GB1814@fixme-laptop.cn.ibm.com> References: <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> <20150921233704.GD970@fixme-laptop.cn.ibm.com> <20150922152540.GO4029@linux.vnet.ibm.com> <20150923000754.GB27867@fixme-laptop.cn.ibm.com> <20150925212904.GC30373@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="CdrF4e02JqNVZeln" Content-Disposition: inline In-Reply-To: <20150925212904.GC30373@linux.vnet.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 --CdrF4e02JqNVZeln Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Sep 25, 2015 at 02:29:04PM -0700, Paul E. McKenney wrote: > On Wed, Sep 23, 2015 at 08:07:55AM +0800, Boqun Feng wrote: > > On Tue, Sep 22, 2015 at 08:25:40AM -0700, Paul E. McKenney wrote: > > > On Tue, Sep 22, 2015 at 07:37:04AM +0800, Boqun Feng wrote: > > > > 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 wrot= e: > > > > > > > > > > On powerpc, we don't need a general memory barrier to a= chieve acquire and > > > > > > > > > > release semantics, so __atomic_op_{acquire,release} can= be implemented > > > > > > > > > > using "lwsync" and "isync". > > > > > > > > >=20 > > > > > > > > > I'm assuming isync+ctrl isn't transitive, so we need to g= et to the 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 ab= out backwards > > > > > > branches for the ctrl+isync construction (none of the current e= xample do, > > > > > > afaict). > > > > > >=20 > > > > >=20 > > > > > Yes.. my care of backwards branches is not quite related to the t= opic, 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 m= ixing > > > > > > ACQUIRE/RELEASE accesses with relaxed accesses (i.e. where an a= ccess from > > > > > > one group reads from an access in the other group). It would be= simplest > > > > > > to say that this doesn't provide any transitivity guarantees, a= nd that > > > > > > an ACQUIRE must always read from a RELEASE if transitivity is r= equired. > > > > > >=20 > > > > >=20 > > > > > Agreed. RELEASE alone doesn't provide transitivity and transitivi= ty is > > > > ^^^^^^^ > > > > This should be ACQUIRE... > > > >=20 > > > > > 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, r= ight? > > >=20 > > > We are going to have to err on the side of strictness, that is, having > > > the documentation place more requirements on the developer than the u= nion > > > of the hardware does. Besides, I haven't heard any recent complaints > > > that memory-barriers.txt is too simple. ;-) > >=20 > > Agreed ;-) > >=20 > > For atomic operations, using isync in ACQUIRE operations does gaurantee > > that a pure RELEASE/ACQUIRE chain provides transitivity. So, again, I > > think we are fine here to use isync in ACQUIRE atomic operations, > > unless you think we need to be more strict, i.e, making ACQUIRE itself > > provide transitivy? >=20 > As I understand it, either isync or lwsync suffices, with the choice > depending on the hardware. The kernel will rewrite itself at boot time > if you use the appropriate macro. ;-) >=20 Yep ;-) Thank you and Will both for your comments. To be honest, I just began to learn about these transitivity and cumulativity things recently. That's a lot of fun to discuss these with you, Peter and Will, and the herdtools you suggested and the N2745 document you wrote really helped a lot. Thank you again! Regards, Boqun --CdrF4e02JqNVZeln Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAABCAAGBQJWBgBuAAoJEEl56MO1B/q42QwH/1kBixM6PrsET/ev+uZboyJC bMSfPvToIS6WoiKHh0dX9RTKtlpvO1xm0IfTV/4r/geeqWrAoIrnaH0LIijH+2gs qgGEFVsYOidoWoQQ9JcbL72CEenWf+1Y6Dijnd/51PFE/1F6vCZYFkoyTWDt0UVO S+FCg1k1ymSmJC/EuFL93oxNVzjeraupTUiEQ2ElXWcfPEjoVpbG9KPnwqN/9Lmb TofEr0lbboSdhaRiXYZw0QJMJV2K8HKybX9aQMerWfuIinF2UemgWiRsaePul3F0 Im7oW+4Y/ZYJrUQOCzIkuoBS1JrVwS/CENf0Ci7xFtEI0LJRqRaS37cp/waPnkk= =0QrP -----END PGP SIGNATURE----- --CdrF4e02JqNVZeln--