From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753188AbbJMOrR (ORCPT ); Tue, 13 Oct 2015 10:47:17 -0400 Received: from mail-pa0-f45.google.com ([209.85.220.45]:34136 "EHLO mail-pa0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752699AbbJMOrQ (ORCPT ); Tue, 13 Oct 2015 10:47:16 -0400 Date: Tue, 13 Oct 2015 22:46:52 +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: <20151013144652.GA14305@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> <20151013143259.GB23991@fixme-laptop.cn.ibm.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="SUOF0GtieIMvvwua" Content-Disposition: inline In-Reply-To: <20151013143259.GB23991@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 --SUOF0GtieIMvvwua Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Oct 13, 2015 at 10:32:59PM +0800, Boqun Feng wrote: > 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 bui= ld > > > them automatically. > >=20 > > The "superfluous barriers" are for the case where the cmpxchg fails, ri= ght? >=20 > Yes. >=20 > > 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 >=20 > Mostly because of the comments in include/linux/atomic.h: >=20 > * 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. >=20 > 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 >=20 > ed2de9f74ecb ("locking/Documentation: Clarify failed cmpxchg() memory ord= ering semantics") >=20 > Because it seems this commit only claims that the barriers in fully > ordered version are conditional. >=20 >=20 > 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: >=20 > unsigned int prev; >=20 > __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"); >=20 > return prev; >=20 >=20 > However, I need to check whether the architecture allows this and any > other problem exists. >=20 > 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 Correction: we can't just put the sync in the middle to implement a fully ordered version. Sorry for that mistake.. Regards, Boqun > version cmpxchg(). Too much needs to investigate then.. >=20 > > (just checking I understand your reasoning). > >=20 >=20 > That actually helps me find a probably better implementation if allowed, > thank you ;-) >=20 > Regards, > Boqun --SUOF0GtieIMvvwua Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAABCAAGBQJWHRlZAAoJEEl56MO1B/q4RJsIAKpd2eHf50kfIiM+LCXT+ham H74krzZDIkDevu7IZxlwlZgDxt/Z46f+mgHJ+mlkb3cx0U812EzbvkLGYi1BDz0c L9Mzspx7Gz8/wnnnYBaxDDiTt6ahWffuBVw8fz4hlq/RZSzPSdl6idqqpKMgM0uC 2/PePTr20WsZQ3i32/a9/zixWYgPr3aKF3K9d7ujPmk9gwT6ib+SVO42j9WcOnv5 EFu1xkBstovOdJGcnSazYK6qpti6yab6osMdI+srwcSWVv2YEFA3bG3lON32OCRR NLNZaU3nwTQe9e1jCzX2OXipP8qIu110v5ATjcCK+oj3X1WoeaJSfYFYLLyB50k= =JI+s -----END PGP SIGNATURE----- --SUOF0GtieIMvvwua--