From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Mon, 24 Nov 2014 22:17:05 +0100 (CET) Received: from e06smtp11.uk.ibm.com ([195.75.94.107]:59104 "EHLO e06smtp11.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S27006812AbaKXVREkPKlb (ORCPT ); Mon, 24 Nov 2014 22:17:04 +0100 Received: from /spool/local by e06smtp11.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 24 Nov 2014 21:16:59 -0000 Received: from d06dlp01.portsmouth.uk.ibm.com (9.149.20.13) by e06smtp11.uk.ibm.com (192.168.101.141) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; Mon, 24 Nov 2014 21:16:57 -0000 Received: from b06cxnps4076.portsmouth.uk.ibm.com (d06relay13.portsmouth.uk.ibm.com [9.149.109.198]) by d06dlp01.portsmouth.uk.ibm.com (Postfix) with ESMTP id B0FD517D8042 for ; Mon, 24 Nov 2014 21:17:11 +0000 (GMT) Received: from d06av04.portsmouth.uk.ibm.com (d06av04.portsmouth.uk.ibm.com [9.149.37.216]) by b06cxnps4076.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id sAOLGu0h59572448 for ; Mon, 24 Nov 2014 21:16:56 GMT Received: from d06av04.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av04.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id sAOLGtd7012024 for ; Mon, 24 Nov 2014 14:16:56 -0700 Received: from oc1450873852.ibm.com (sig-9-79-90-165.de.ibm.com [9.79.90.165]) by d06av04.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVin) with ESMTP id sAOLGsUZ011983; Mon, 24 Nov 2014 14:16:54 -0700 Message-ID: <5473A046.2020901@de.ibm.com> Date: Mon, 24 Nov 2014 22:16:54 +0100 From: Christian Borntraeger User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: Linus Torvalds CC: David Howells , Alexei Starovoitov , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" , linux-mips , linux-x86_64@vger.kernel.org, linux-s390 , Paolo Bonzini , Paul McKenney , Ingo Molnar , Catalin Marinas , Will Deacon Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types References: <1416834210-61738-1-git-send-email-borntraeger@de.ibm.com> <1416834210-61738-8-git-send-email-borntraeger@de.ibm.com> <15567.1416835858@warthog.procyon.org.uk> <547381D7.2070404@de.ibm.com> <12209.1416859494@warthog.procyon.org.uk> <54739AB2.8030002@de.ibm.com> In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14112421-0005-0000-0000-00000234E678 Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 44409 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: borntraeger@de.ibm.com Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips Am 24.11.2014 um 22:02 schrieb Linus Torvalds: > On Mon, Nov 24, 2014 at 12:53 PM, Christian Borntraeger > wrote: >> >> That looks like a lot of changes all over ACCESS_ONCE -> ASSIGN_ONCE: >> git grep "ACCESS_ONCE.*=.*" >> gives me 200 placea not in Documentation. > > Yeah, that's a bit annoying. > > How about a combination of the two: > > - accept the fact that right now ACCESS_ONCE() is fairly widespread > (even for writing) > > - but also admit that we'd be better off with a nicer interface > > and make the solution be: > > - make ACCESS_ONCE() only work on scalars, and deprecate it > > - add new "read_once()" and "write_once()" interfaces that *do* work > on (appropriately sized) structures and unions, and start migrating > things over. In particular, start with the ones that can no longer use > ACCESS_ONCE() because they aren't scalar.. > > That second point would make the conversion patches actually easier to > read. Instead of this: > > static inline int arch_spin_is_locked(arch_spinlock_t *lock) > { > - struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets); > + arch_spinlock_t tmp = {}; > > - return tmp.tail != tmp.head; > + tmp.head_tail =ACCESS_ONCE(lock->head_tail); > + return tmp.tickets.tail != tmp.tickets.head; > } > > which isn't *complex*, but is also not an obvious conversion, we'd have just > > static inline int arch_spin_is_locked(arch_spinlock_t *lock) > { > - struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets); > - struct __raw_tickets tmp = read_once(lock->tickets); > > return tmp.tail != tmp.head; > } > > which is a much simpler and more obvious change. > > And then we could slowly try to migrate existing ACCESS_ONCE() users > over (particularly writers). > > Hmm? Too much? I will give it a try. I will start with Alexei's version for ACCESS_ONCE and your snippets to build read_once and write_once. The only open question is, what to do with the "too large" accesses. Pauls initial patch showed several places, e.g. kernel/sched/fair.c accessing an u64 even on 32bit: [...] age_stamp = ACCESS_ONCE(rq->age_stamp); avg = ACCESS_ONCE(rq->rt_avg); [...] I think I will simply not touch those... Christian