From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Mon, 24 Nov 2014 21:53:18 +0100 (CET) Received: from e06smtp12.uk.ibm.com ([195.75.94.108]:45157 "EHLO e06smtp12.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S27011494AbaKXUxRJ4vtR (ORCPT ); Mon, 24 Nov 2014 21:53:17 +0100 Received: from /spool/local by e06smtp12.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 24 Nov 2014 20:53:11 -0000 Received: from d06dlp02.portsmouth.uk.ibm.com (9.149.20.14) by e06smtp12.uk.ibm.com (192.168.101.142) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; Mon, 24 Nov 2014 20:53:08 -0000 Received: from b06cxnps3075.portsmouth.uk.ibm.com (d06relay10.portsmouth.uk.ibm.com [9.149.109.195]) by d06dlp02.portsmouth.uk.ibm.com (Postfix) with ESMTP id DB2252190046 for ; Mon, 24 Nov 2014 20:52:40 +0000 (GMT) Received: from d06av08.portsmouth.uk.ibm.com (d06av08.portsmouth.uk.ibm.com [9.149.37.249]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id sAOKr8dP18022796 for ; Mon, 24 Nov 2014 20:53:08 GMT Received: from d06av08.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av08.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id sAOKr7hm012215 for ; Mon, 24 Nov 2014 13:53:08 -0700 Received: from oc1450873852.ibm.com (sig-9-79-90-165.de.ibm.com [9.79.90.165]) by d06av08.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVin) with ESMTP id sAOKr6fr012192; Mon, 24 Nov 2014 13:53:06 -0700 Message-ID: <54739AB2.8030002@de.ibm.com> Date: Mon, 24 Nov 2014 21:53:06 +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 , David Howells CC: 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> 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: 14112420-0009-0000-0000-0000021E5B54 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: 44406 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 21:34 schrieb Linus Torvalds: > On Mon, Nov 24, 2014 at 12:04 PM, David Howells wrote: >> >> Reserve ACCESS_ONCE() for reading and add an ASSIGN_ONCE() or something like >> that for writing? > > I wouldn't mind that. We've had situations where reading and writing > isn't really similar - like alpha where reading a byte is atomic, but > writing one isn't. > > Then we could also make it have the "get_user()/put_user()" kind of > semantics - .and then use the same "sizeopf()" tricks that we use for > get_user/put_user. > > That would actually work around the gcc bug a completely different way: > > #define ACCESS_ONCE(p) \ > ({ typeof(*p) __val; __read_once_size(p, &__val, sizeof(__val)); __val; }) > > and then we can do things like this: > > static __always_inline void __read_once_size(volatile void *p, void > *res, int size) > { > switch (size) { > case 1: *(u8 *)res = *(volatile u8 *)p; break; > case 2: *(u16 *)res = *(volatile u16 *)p; break; > case 4: *(u32 *)res = *(volatile u32 *)p; break; > #ifdef CONFIG_64BIT > case 8: *(u64 *)res = *(volatile u64 *)p; break; > #endif > } > } > > and same for ASSIGN_ONCE(val, p). > > That also hopefully avoids the whole "oops, gcc has a bug", because > the actual volatile access is always done using a scalar type, even if > the type of "__val" may in fact be a structure. > > Christian, how painful would that be? Sorry to try to make you do a > totally different approach.. That looks like a lot of changes all over ACCESS_ONCE -> ASSIGN_ONCE: git grep "ACCESS_ONCE.*=.*" gives me 200 placea not in Documentation. Then there is still the 64bit accesses on 32bit via ACCESS_ONCE problem, which we could detect with a default cause in your code. We would need to audit and fix all places :-/ So the last proposal from Alexei, seems easier (for me at least :-) )