From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932254AbcCILNV (ORCPT ); Wed, 9 Mar 2016 06:13:21 -0500 Received: from smtprelay.synopsys.com ([198.182.47.9]:44230 "EHLO smtprelay.synopsys.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753156AbcCILNI (ORCPT ); Wed, 9 Mar 2016 06:13:08 -0500 Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic To: Peter Zijlstra References: <1457447457-25878-1-git-send-email-vgupta@synopsys.com> <56DEF3D3.6080008@synopsys.com> <56DFC604.6070407@synopsys.com> <20160309101349.GJ6344@twins.programming.kicks-ass.net> <20160309103143.GF25010@twins.programming.kicks-ass.net> CC: Christoph Lameter , , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , "Noam Camus" , , , , , "James E.J. Bottomley" , Helge Deller , "linux-arch@vger.kernel.org" Newsgroups: gmane.linux.kernel.stable,gmane.linux.kernel.mm,gmane.linux.kernel,gmane.linux.kernel.arc,gmane.linux.kernel.cross-arch From: Vineet Gupta Message-ID: <56E0052B.3080304@synopsys.com> Date: Wed, 9 Mar 2016 16:42:43 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <20160309103143.GF25010@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.12.197.157] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday 09 March 2016 04:01 PM, Peter Zijlstra wrote: > On Wed, Mar 09, 2016 at 11:13:49AM +0100, Peter Zijlstra wrote: >> --- >> Subject: bitops: Do not default to __clear_bit() for __clear_bit_unlock() >> >> __clear_bit_unlock() is a special little snowflake. While it carries the >> non-atomic '__' prefix, it is specifically documented to pair with >> test_and_set_bit() and therefore should be 'somewhat' atomic. >> >> Therefore the generic implementation of __clear_bit_unlock() cannot use >> the fully non-atomic __clear_bit() as a default. >> >> If an arch is able to do better; is must provide an implementation of >> __clear_bit_unlock() itself. >> > > FWIW, we could probably undo this if we unified all the spinlock based > atomic ops implementations (there's a whole bunch doing essentially the > same), and special cased __clear_bit_unlock() for that. > > Collapsing them is probably a good idea anyway, just a fair bit of > non-trivial work to figure out all the differences and if they matter > etc.. Indeed I thought about this when we first did the SMP port. The only issue was somewhat more generated code with the hashed spinlocks (vs. my dumb 2 spin locks - which as I see now will also cause false sharing - likely ending up in the same cache line), but I was more of a micro-optimization freak then than I'm now :-) So yeah I agree !