From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757761AbbEWJXl (ORCPT ); Sat, 23 May 2015 05:23:41 -0400 Received: from hofr.at ([212.69.189.236]:57968 "EHLO mail.hofr.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757301AbbEWJXi (ORCPT ); Sat, 23 May 2015 05:23:38 -0400 Date: Sat, 23 May 2015 11:23:36 +0200 From: Nicholas Mc Guire To: Oleg Nesterov Cc: Peter Zijlstra , Nicholas Mc Guire , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [PATCH] locking: type cleanup when accessing fast_read_ctr Message-ID: <20150523092336.GA1930@opentech.at> References: <1431971282-21412-1-git-send-email-hofrat@osadl.org> <20150519111105.GB3644@twins.programming.kicks-ass.net> <20150519112513.GA1337@opentech.at> <20150520174431.GB6831@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150520174431.GB6831@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 20 May 2015, Oleg Nesterov wrote: > On 05/19, Nicholas Mc Guire wrote: > > > > I assumed it would not matter but did not see a simple way of getting it > > type clean with unsigned either mainly due to the atomic_t being int and > > val in update_fast_ctr() being passed as -1. > > Perhaps clear_fast_ctr() should have a comment to explain why it returns > "int"... even if "unsigned" should work too. > Might not be into c99 standard far enough but from reviewing 6.5/J.2 I do not see this argument here. The "well defined modulo 2**n" behavior for unsigned int can be found stated in a few places - but not in the c99 standard for arithmetic overflow. The "well defined overflow behavior" as far as I understand c99, *only* applies to left shift operations when overflowing - see 6.5.7 " Bitwise shift operators" -> Semantics -> 4) - further for the counter problem this well defined behavior is of little help as the sum would be wrong in both cases. I still do not see the point in the implicit/automatic type conversion here and why that should be an advantage - could somone point me to the right c99 clauses ? thx! hofrat