From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753099AbbJMOd0 (ORCPT ); Tue, 13 Oct 2015 10:33:26 -0400 Received: from mail-pa0-f44.google.com ([209.85.220.44]:35219 "EHLO mail-pa0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752295AbbJMOdY (ORCPT ); Tue, 13 Oct 2015 10:33:24 -0400 Date: Tue, 13 Oct 2015 22:32:59 +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 , Davidlohr Bueso Subject: Re: [PATCH v3 6/6] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg_* variants Message-ID: <20151013143259.GB23991@fixme-laptop.cn.ibm.com> References: <1444659246-24769-1-git-send-email-boqun.feng@gmail.com> <1444659246-24769-7-git-send-email-boqun.feng@gmail.com> <20151013132404.GI21550@arm.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="OwLcNYc0lM97+oe1" Content-Disposition: inline In-Reply-To: <20151013132404.GI21550@arm.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 --OwLcNYc0lM97+oe1 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Oct 13, 2015 at 02:24:04PM +0100, Will Deacon wrote: > On Mon, Oct 12, 2015 at 10:14:06PM +0800, Boqun Feng wrote: > > Implement cmpxchg{,64}_relaxed and atomic{,64}_cmpxchg_relaxed, based on > > which _release variants can be built. > >=20 > > To avoid superfluous barriers in _acquire variants, we implement these > > operations with assembly code rather use __atomic_op_acquire() to build > > them automatically. >=20 > The "superfluous barriers" are for the case where the cmpxchg fails, righ= t? Yes. > And you don't do the same thing for release, because you want to avoid a > barrier in the middle of the critical section? >=20 Mostly because of the comments in include/linux/atomic.h: * For compound atomics performing both a load and a store, ACQUIRE * semantics apply only to the load and RELEASE semantics only to the * store portion of the operation. Note that a failed cmpxchg_acquire * does -not- imply any memory ordering constraints. so I thought only the barrier in cmpxchg_acquire() is conditional, and the barrier in cmpxchg_release() is not. Maybe we'd better call it out that cmpxchg *family* doesn't have any order guarantee if cmp fails, as a complement of ed2de9f74ecb ("locking/Documentation: Clarify failed cmpxchg() memory order= ing semantics") Because it seems this commit only claims that the barriers in fully ordered version are conditional. If cmpxchg_release doesn't have order guarantee when failed, I guess I can implement it with a barrier in the middle as you mentioned: unsigned int prev; __asm__ __volatile__ ( "1: lwarx %0,0,%2 =09 cmpw 0,%0,%3\n\ bne- 2f\n" PPC_RELEASE_BARRIER " stwcx. %4,0,%2\n\ bne- 1b" "\n\ 2:" : "=3D&r" (prev), "+m" (*p) : "r" (p), "r" (old), "r" (new) : "cc", "memory"); return prev; However, I need to check whether the architecture allows this and any other problem exists. Besides, I don't think it's a good idea to do the "put barrier in the middle" thing in this patchset, because that seems a premature optimization and if we go further, I guess we can also replace the PPC_RELEASE_BARRIER above with a "sync" to implement a fully ordered version cmpxchg(). Too much needs to investigate then.. > (just checking I understand your reasoning). >=20 That actually helps me find a probably better implementation if allowed, thank you ;-) Regards, Boqun --OwLcNYc0lM97+oe1 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAABCAAGBQJWHRYYAAoJEEl56MO1B/q4P9wIALikvsbiu8Aw3Ahivu+FBaLE oNAN+8CPOOiV/uchhqHs+JjcjYE4outU44zuIhbVeWYd2KAVBSxJMk3ZWiQEGdA4 dSLbS/TGUHeMIGRdGC4AfU9MQV3ybRayPRWUlzHiiSHaWtHkkgkS5vVB/tjXIwV4 FMNsEkPuNf2hcDRAB9ky86I2mROjUyolC8DyC32MqapC5wDrrdxoOX0gUB1RbB7U yYCM0ZN6xQlnP50oVIhQJyJmYBockJFl9e2at2VqYfhR9SUpxsSt/lpUGyp+H8Sa CtUmasniAzIJphMqvPUsXT5SX5H9dWEuo2sjErdzT3vLzYVg05kP/cW4VYaoUms= =kHh8 -----END PGP SIGNATURE----- --OwLcNYc0lM97+oe1--