From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756006AbbHDKdL (ORCPT ); Tue, 4 Aug 2015 06:33:11 -0400 Received: from e06smtp13.uk.ibm.com ([195.75.94.109]:59907 "EHLO e06smtp13.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755496AbbHDKdJ (ORCPT ); Tue, 4 Aug 2015 06:33:09 -0400 X-Helo: d06dlp02.portsmouth.uk.ibm.com X-MailFrom: borntraeger@de.ibm.com X-RcptTo: linux-kernel@vger.kernel.org Message-ID: <55C094E0.7020406@de.ibm.com> Date: Tue, 04 Aug 2015 12:33:04 +0200 From: Christian Borntraeger User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.8.0 MIME-Version: 1.0 To: Peter Zijlstra CC: Andrey Konovalov , Ingo Molnar , Paul McKenney , linux-kernel@vger.kernel.org Subject: Re: [PATCH] compiler.h: cast away attributes in WRITE_ONCE magic References: <1438674948-38310-1-git-send-email-borntraeger@de.ibm.com> <1438674948-38310-2-git-send-email-borntraeger@de.ibm.com> <20150804092027.GN25159@twins.programming.kicks-ass.net> In-Reply-To: <20150804092027.GN25159@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15080410-0013-0000-0000-000004E991B8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am 04.08.2015 um 11:20 schrieb Peter Zijlstra: > On Tue, Aug 04, 2015 at 09:55:48AM +0200, Christian Borntraeger wrote: >> kernel build bot showed a warning triggered by commit >> 76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in >> smp_store_release()/smp_load_acquire()"). Turns out that sparse >> does not like WRITE_ONCE accessing elements from the (sparse) >> rcu address space. >> >> fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces) >> fs/afs/inode.c:448:9: expected struct afs_permits *__val >> fs/afs/inode.c:448:9: got void [noderef] * >> >> Solution is to force cast away the sparse attributes for the initializer >> of the union in WRITE_SAME. As this now gets too long, lets split > > WRITE_ONCE, right? Indeed, guess what area I was looking into before....;-) Shall I respin or can you fixup. > >> the macro. >> >> Signed-off-by: Christian Borntraeger >> --- >> include/linux/compiler.h | 7 ++++++- >> 1 file changed, 6 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/compiler.h b/include/linux/compiler.h >> index e08a6ae..c836eb2 100644 >> --- a/include/linux/compiler.h >> +++ b/include/linux/compiler.h >> @@ -252,7 +252,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s >> ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; }) >> >> #define WRITE_ONCE(x, val) \ >> - ({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; }) >> +({ \ >> + union { typeof(x) __val; char __c[1]; } __u = \ >> + { .__val = (__force typeof(x)) (val) }; \ >> + __write_once_size(&(x), __u.__c, sizeof(x)); \ >> + __u.__val; \ >> +}) > > Thanks! > > READ_ONCE() doesn't have a similar problem because it only has the one > input type, which automagically becomes the output type, right? Good question. The assigment that failed was the initialization of the temporary union, which was introduced to handle const pointers. READ_ONCE does not have such assignment so this thing should be safe. I cant say right now if the macro return value causes issues somewhere else, e.g. does code want to have values __rcu tagged? I have checked some of the kbuild output and all new things seem to be fixed so this patch itself is probably an improvement (pending kbuild coverage). Having a testcase that works with ACCESS_ONCE but fails with READ_ONCE would be a good start to see if we need to change READ_ONCE further. Christian